The Problems of Philosophy

by Bertrand Russell

Troy Shu
Troy Shu
Updated at: May 29, 2024
The Problems of Philosophy
The Problems of Philosophy

Explore the philosophical challenges of reality, knowledge, and the limits of metaphysics in this comprehensive book summary. Discover insights to broaden your mind and enhance critical thinking.

What are the big ideas?

Reality Behind Appearances

The book explores the distinction between appearance and reality, emphasizing the philosophical challenge in determining the true essence of objects beyond sensory perceptions.

Berkeley's Idealism Challenged

Unique analysis of Berkeley's idealism, critiquing his view that matter doesn't exist outside of perception and offering a counter-perspective that aligns with common sense beliefs in physical reality.

Induction and Knowledge Expansion

The book digs into the problem of induction, proposing a principle that relates past associations to future expectations, and questioning the reliability of such inferences as bases for knowledge.

Limits of Metaphysics

A critical examination of the claims of metaphysics, arguing against the possibility of definitive a priori knowledge about the universe and stressing the importance of empirical investigation.

Navigating the World of Universals

Discusses the concept of universals, how they differ from particular sensory experiences, and their role in forming a priori knowledge, presenting a nuanced view of how we understand general principles.

Philosophy's Intrinsic Value

Asserts the value of philosophy not in practical utility but in its role in broadening the mind, enhancing critical thinking, and promoting a deeper understanding of fundamental, existential queries.

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Reality Behind Appearances

The central insight is the challenge of discerning the true nature of reality beyond our immediate sensory experiences. The book grapples with the philosophical question of whether the objects we perceive through our senses accurately reflect the underlying reality, or if there is a deeper essence that lies hidden behind the appearances.

This distinction between appearance and reality is a fundamental problem in epistemology - the study of knowledge and how we can know what is true. On one hand, we have direct sensory knowledge of the world through our sight, touch, hearing, and other senses. But on the other hand, there is the possibility that these sensory perceptions do not fully capture the true nature of the objects themselves.

The book explores various perspectives on this issue. It considers the view that our senses may be unreliable guides to reality, and that there may be an underlying "physical object" that is distinct from the sensory "sense-data" we directly experience. Resolving this tension between appearance and reality is crucial for developing a comprehensive understanding of the world and our place in it.

Here are some key examples from the context that illustrate the distinction between appearance and reality:

  • The real table, if there is one, is not the same as what we immediately experience by sight, touch, or hearing. The real table, if it exists, is not immediately known to us at all, but must be inferred from what is immediately known through our senses.

  • The sense-data we experience, such as colors, sounds, hardness, etc., cannot be directly equated with properties of the physical table itself. The sense-data are "signs" of some underlying property of the table, but are not the table itself.

  • Berkeley argues that the existence of "matter" as something radically different from the mind or ideas in the mind cannot be proven. The "real" table, if it exists, must be an idea in the mind of God, rather than a physical object independent of the mind.

  • The context discusses the philosophical challenge in determining the true nature of objects beyond our immediate sensory perceptions. Our knowledge of the physical world is mediated through sense-data, which may not directly reflect the underlying reality.

Key terms and concepts:

  • Sense-data: The immediate objects of sensory experience, such as colors, sounds, hardness, etc.
  • Physical object: The "real" table that may exist independently of our perceptions.
  • Matter: The collection of all physical objects, which Berkeley argues cannot be proven to exist independently of the mind.

Berkeley's Idealism Challenged

Berkeley's idealism is challenged by the recognition that our perception of objects is distinct from the objects themselves. While our senses may shape our experience of an object, this does not prove the object is purely mental or exists only in the mind. The color of a table, for instance, depends on the relationship between our eyes and the physical table, but this does not mean the color is inherently mental.

The fallacy in Berkeley's argument is confusing the act of perception with the object perceived. The mental act of apprehending an object is undoubtedly mental, but the object itself is not necessarily so. Equating the two leads to the unsupported conclusion that all objects of perception must exist only in the mind.

This distinction between the mental and the physical is crucial, as it preserves our common-sense belief in a physical reality that exists independently of our perception of it. Dismissing this physical reality as mere illusion or mental construct undermines our ability to acquire genuine knowledge about the world. A more nuanced view acknowledges both the role of the mind in shaping experience and the existence of an external, physical realm.

Key Insight: Berkeley's Idealism Challenged

The passage challenges Berkeley's idealist view that matter does not exist outside of perception, offering a counter-perspective that aligns with common sense beliefs in physical reality:

  • The passage states that Berkeley believes "whatever can be apprehended must be in our minds" due to an "unconscious equivocation" where he confuses the mental act of apprehension with the thing being apprehended.
  • It argues that this view is flawed, as "the thing apprehended is in any sense mental" is not proven by Berkeley's arguments. The previous arguments only showed that the existence of sense-data depends on the relation between our senses and the physical object, not that the object itself is mental.
  • The passage suggests that saying "ideas are in the mind" is a "mere tautology" if it just means the object is being apprehended, and that we must admit "what, in this sense, is in the mind, may nevertheless be not mental."
  • It contends that Berkeley's grounds for idealism "may be dismissed" and that there may be other grounds for rejecting the existence of matter, which the passage aims to examine further.
  • The passage highlights how Berkeley and other idealist philosophers, while denying matter as opposed to mind, "nevertheless, in another sense, admit matter" by agreeing there is a "real table" that exists independently, even if they disagree on its nature.

Induction and Knowledge Expansion

The passage explores the problem of induction - how we can gain knowledge about the world beyond our immediate experiences. It notes that much of our everyday beliefs and reasoning rely on inductive inferences, where we generalize from past observations to make predictions about the future.

However, the author questions the reliability of such inductive reasoning. While inductive principles like "the food we eat is likely nourishing, not poisonous" are constantly used in our thinking, the author argues there is no way to conclusively prove these principles through simpler, self-evident principles. The inductive principle itself seems to be a foundational assumption, rather than something that can be deduced.

This raises challenges for how we can be certain about knowledge that goes beyond our direct experiences. The author suggests that in addition to inductive knowledge, there are also self-evident truths - logical principles and mathematical facts that we can know with certainty, without requiring proof from more basic premises. Exploring the nature and scope of these self-evident truths becomes an important task for understanding the limits and sources of human knowledge.

Overall, the passage highlights the tension between the expansive, inductive nature of much of our knowledge, and the need to ground that knowledge in more secure, self-evident foundations. Resolving this tension is crucial for developing a robust epistemology - a theory of how we can know what is true.

Here are the key examples from the context that support the insight about induction and knowledge expansion:

  • The book states that "in most questions of daily life, such as whether our food is likely to be nourishing and not poisonous, we shall be driven back to the inductive principle, which we discussed in Chapter VI." This indicates that induction is a key principle used to make everyday judgments.

  • However, the book notes that beyond the inductive principle, "there seems to be no further regress. The principle itself is constantly used in our reasoning, sometimes consciously, sometimes unconsciously; but there is no reasoning which, starting from some simpler self-evident principle, leads us to the principle of induction as its conclusion." This suggests that the inductive principle itself cannot be further justified through deduction from more basic principles.

  • The book also states that the same holds true for other "logical principles" - "Their truth is evident to us, and we employ them in constructing demonstrations; but they themselves, or at least some of them, are incapable of demonstration." This indicates that there are certain foundational principles that cannot be proven through further reasoning.

In summary, the key examples illustrate how induction and other logical principles are fundamental to our reasoning and knowledge, but themselves cannot be further justified through deduction from more basic principles. This highlights the limitations of expanding knowledge solely through deductive reasoning.

Limits of Metaphysics

The limits of metaphysics are clear. Attempts to use pure reason to definitively prove the fundamental dogmas of religion, the rationality of the universe, the illusory nature of matter, or the unreality of evil are ultimately futile. Such a priori metaphysical reasoning is incapable of surviving critical scrutiny.

Philosophers like Hegel have tried to reconstruct the entire universe through pure thought, arguing that every piece of reality is inherently incomplete and must be understood in relation to the whole. However, this approach inevitably leads to contradictions that can only be resolved by introducing new, less incomplete ideas. But these new ideas also prove to be incomplete, perpetuating an endless cycle of thesis, antithesis, and synthesis.

True knowledge about the universe as a whole cannot be obtained through metaphysics alone. Empirical investigation is essential. While coherent philosophical systems may increase the probability of certain beliefs, they can never transform probable opinions into indubitable knowledge. The limits of philosophical knowledge must be recognized and respected.

Here are key examples from the context that support the key insight about the limits of metaphysics:

  • The author argues that while some of our knowledge comes from intuitive self-evident truths, much of our knowledge depends on empirical investigation and inductive reasoning. As the author states, "Principles such as the law of gravitation are proved, or rather are rendered highly probable, by a combination of experience with some wholly a priori principle, such as the principle of induction."

  • The author is skeptical of the ability of metaphysics to provide definitive a priori knowledge about the universe as a whole, stating that "our inquiry has not revealed any such knowledge, and therefore, as regards the special doctrines of the bolder metaphysicians, has had a mainly negative result."

  • The author critiques Kant's attempt to ground a priori knowledge in the structure of the human mind, arguing that this does not account for the "certainty that the facts must always conform to logic and arithmetic." The author states that "Our nature is as much a fact of the existing world as anything, and there can be no certainty that it will remain constant."

  • The author emphasizes the importance of empirical investigation, stating that "Philosophical knowledge, if what has been said above is true, does not differ essentially from scientific knowledge; there is no special source of wisdom which is open to philosophy but not to science."

In summary, the key examples highlight the author's skepticism towards metaphysical claims of definitive a priori knowledge, and the emphasis on the need for empirical investigation and inductive reasoning to gain reliable knowledge about the world.

Navigating the World of Universals

Universals are abstract concepts that exist independently of particular sensory experiences. They are timeless and unchanging, unlike the fleeting, vague world of physical objects and mental states. Universals include qualities like whiteness, relations like "before and after", and logical principles.

We can gain knowledge by acquaintance of certain universals, such as sensory qualities and basic relations, by abstracting from our experiences. This allows us to understand general truths, like "two and two are four", without needing to know every particular instance.

This a priori knowledge of universals and their relations is the foundation for logical reasoning and mathematical proofs. It does not depend on specific experiences, but on our direct apprehension of the universal concepts involved.

However, not all universals can be known through acquaintance. Some must be grasped indirectly, through description. And there may be universals that remain unknown to us entirely. Carefully distinguishing what we can know directly from what requires inference is key to navigating this realm of abstract, timeless entities.

Here are the key insights and supporting examples from the context:

  • Universals vs. Particulars: Universals are timeless, unchangeable entities that exist independently of being thought about, in contrast to the fleeting, vague world of particular sensory experiences and physical objects.

    • The author states that universals "subsist or have being, where 'being' is opposed to 'existence' as being timeless" and that the "world of universals...may also be described as the world of being."
    • In contrast, the "world of existence is fleeting, vague, without sharp boundaries, without any clear plan or arrangement, but it contains all thoughts and feelings, all the data of sense, and all physical objects."
  • Acquaintance with Universals: We can have direct acquaintance with certain universals, such as sensory qualities (e.g. whiteness, redness) and relations (e.g. spatial, temporal, and resemblance relations).

    • The author states we can become "acquainted with any other universal of the same sort" as sensory qualities by "seeing many white patches" and abstracting the common whiteness.
    • Similarly, we can become acquainted with spatial relations like "being to the left of" by perceiving them in sense-data, and with temporal relations like "before and after" through perception and memory.
  • Universals and A Priori Knowledge: All a priori knowledge, such as logical and mathematical principles, deals exclusively with the relations between universals, not with particular sensory experiences.

    • The author states that the proposition "two and two are four" "states a relation between the universal 'two' and the universal 'four'", and that "All a priori knowledge deals exclusively with the relations of universals."
    • This allows us to understand a priori truths without needing to know or experience every particular instance, since they concern the universal relations themselves.
  • Universals as Independent of Mind: Universals, including relations, have an objective, mind-independent existence, contrary to the views of empiricist philosophers like Berkeley and Hume.

    • The author argues that relations like "north of" must subsist independently of being thought about, since the fact that "Edinburgh is north of London" is true regardless of whether anyone knows it.
    • Rejecting universals would lead to problematic theories, like having to posit a distinct resemblance for each pair of similar particulars, which the author sees as an unacceptable view.

Philosophy's Intrinsic Value

The true value of philosophy lies not in its practical utility, but in its ability to broaden the mind and enhance critical thinking. Philosophy encourages us to question the familiar and obvious, to explore unfamiliar possibilities, and to grapple with fundamental, existential questions about the nature of the universe and our place in it.

By subjecting our common beliefs and prejudices to rigorous examination, philosophy frees us from the tyranny of custom. It exposes the limitations of our knowledge and the uncertainty underlying even our most basic convictions. This process may diminish our feeling of certainty, but it also increases our understanding of the vast realm of what may be possible.

The true value of philosophy, then, is not to be found in any definitive set of answers, but in its power to expand our horizons, to cultivate a spirit of wonder, and to deepen our engagement with the most profound questions facing humanity. It is a discipline that enlarges the self by encouraging us to transcend the narrow confines of our private interests and to contemplate the grandeur of the universe as a whole.

Here are examples from the context that support the key insight about the intrinsic value of philosophy:

  • The man who has no "tincture of philosophy" goes through life "imprisoned in the prejudices derived from common sense, from the habitual beliefs of his age or his nation." Philosophy frees the mind from the "tyranny of custom" and shows "familiar things in an unfamiliar aspect."

  • Philosophic contemplation does not divide the universe into "friends and foes, helpful and hostile, good and bad" but views the whole "impartially." This "enlargement of Self" is not obtained when we try to show the world is similar to the Self, but when we "adapt the Self to the characters which it finds in its objects."

  • Philosophy "keeps alive our sense of wonder by showing familiar things in an unfamiliar aspect." It suggests "many possibilities which enlarge our thoughts and free them from the tyranny of custom."

  • While philosophy cannot provide "definite answers" to its fundamental questions, it is important that it "continue the consideration of such questions, to make us aware of their importance, to examine all the approaches to them, and to keep alive that speculative interest in the universe which is apt to be killed by confining ourselves to definitely ascertainable knowledge."

The key terms and concepts illustrated here are the role of philosophy in:

  • Freeing the mind from prejudices and custom
  • Promoting impartial, contemplative understanding
  • Expanding the self and sense of wonder
  • Sustaining inquiry into fundamental, existential questions


Let's take a look at some key quotes from "The Problems of Philosophy" that resonated with readers.

Some care is needed in using Descartes' argument. "I think, therefore I am" says rather more than is strictly certain. It might seem as though we are quite sure of being the same person to-day as we were yesterday, and this is no doubt true in some sense. But the real Self is as hard to arrive at as the real table, and does not seem to have that absolute, convincing certainty that belongs to particular experiences.

The notion of a consistent self is an illusion. Our sense of identity is fragmented and fleeting, making it challenging to pinpoint a fixed, essential self. Like trying to grasp the true nature of an object, our understanding of ourselves is subjective and uncertain. This ambiguity undermines the idea of a stable, enduring self.

Is there any knowledge in the world which is so certain that no reasonable man could doubt it?

Can we be absolutely sure about anything in life? Is there any truth that is universally accepted, beyond a shadow of a doubt? The question highlights the limitations of human knowledge, suggesting that even the most seemingly certain facts can be subject to scrutiny and debate.

Philosophy, though unable to tell us with certainty what is the true answer to the doubts which it raises, is able to suggest many possiblities which enlarge our thoughts and free them from the tyranny of custom. Thus, while diminishing our feeling of certainty as to what things are, it greatly increases our knowledge as to what the may be; it removes the somewhat arrogant dogmatism of those who have never travelled into the region of liberating doubt, and it keeps alive our sense of wonder by showing familar things in an unfamilar aspect

Philosophy expands our understanding by presenting various possibilities, freeing us from the constraints of conventional thinking. While it may reduce our sense of certainty, it significantly increases our knowledge of what could be. This process fosters a sense of wonder, as familiar things are viewed from a new and intriguing perspective. Ultimately, it humbles our thinking, replacing dogmatic confidence with open-minded curiosity.

Comprehension Questions

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How well do you understand the key insights in "The Problems of Philosophy"? Find out by answering the questions below. Try to answer the question yourself before revealing the answer! Mark the questions as done once you've answered them.

1. What is the challenge in determining the relationship between our sensory experiences and the true nature of objects?
2. How do sense-data relate to physical objects in the philosophical context of appearance versus reality?
3. What does Berkeley postulate about the existence of matter and its relationship with the mind?
4. What is the fundamental error in confusing the act of perception with the object perceived?
5. How does acknowledging the distinction between mental acts and physical objects preserve our belief in a physical reality?
6. Why is the notion that objects are purely mental or exist solely in the mind considered a fallacy?
7. What is the problem of induction in reasoning about the world?
8. How does inductive reasoning impact our everyday decision-making?
9. Why can't the principle of induction be justified by more basic self-evident truths?
10. What implications does the lack of justification for inductive reasoning have on our knowledge system?
11. What is the effectiveness of using pure reason to prove fundamental religious or metaphysical dogmas?
12. How does reliance solely on a priori reasoning impact metaphysical conclusions about the universe?
13. Why can't metaphysical systems provide absolute certainty about the nature of the universe?
14. What is the role of empirical investigation in gaining knowledge about the universe according to the discussed viewpoint?
15. How are philosophical systems similar to and different from scientific knowledge in terms of gaining reliable knowledge?
16. What are universals, and how do they differ from particulars?
17. How can we gain knowledge of universals?
18. What role do universals play in a priori knowledge?
19. Why are some universals considered beyond direct acquaintance?
20. What is the primary benefit of engaging in philosophical thought, despite it not offering concrete answers to complex queries?
21. How does philosophy contribute to freeing an individual from the confines of habitual and prejudicial thinking?
22. What role does philosophy play in altering one's perception of familiar concepts?
23. Describe how philosophical inquiry differs in its approach to understanding the universe compared to other forms of inquiry?
24. Why is it important for philosophy to continue engaging with its fundamental questions without providing definite answers?

Action Questions

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"Knowledge without application is useless," Bruce Lee said. Answer the questions below to practice applying the key insights from "The Problems of Philosophy". Mark the questions as done once you've answered them.

1. How can you use the concept of appearance versus reality to challenge your assumptions and improve your critical thinking?
2. In what ways can understanding the difference between sense-data and physical objects enhance your interactions or judgments in social situations?
3. How can distinguishing between perception and physical reality enhance your approach to problem-solving in professional or personal settings?
4. How can you use inductive reasoning to evaluate the reliability of new information and make everyday decisions more effectively?
5. How can you apply empirical methods to test and refine your personal beliefs or ideas about the world?
6. How can you apply the concept of universals to enhance your understanding and communication of abstract ideas in daily conversations or presentations?
7. In what ways can you utilize the understanding of universals to improve problem-solving strategies in your professional or personal life?
8. How can you apply philosophical thinking to challenge and expand your understanding of everyday issues?

Chapter Notes


Here are the key takeaways from the chapter:

  • Certainty of Knowledge: The chapter questions whether there is any knowledge in the world that is so certain that no reasonable person could doubt it. This is a difficult question that lies at the heart of philosophical inquiry.

  • Appearance vs. Reality: There is a distinction between the "appearance" of things, which is what we directly perceive through our senses, and the "reality" of things, which may be different from their appearance. This distinction causes much confusion and difficulty in philosophy.

  • Sense-Data and Sensation: "Sense-data" refers to the immediate objects of sensation, such as colors, sounds, and textures. "Sensation" refers to the experience of being aware of these sense-data. The relationship between sense-data and the real, physical objects they are associated with is a key philosophical problem.

  • Berkeley's Idealism: The philosopher Berkeley argued that there is no such thing as matter, and that the world consists only of minds and their ideas. He denied the existence of a "real table" independent of perception, and instead saw the table as an idea in the mind of God.

  • Other Idealist Views: Some other philosophers, while agreeing with Berkeley that there is no "matter" as commonly conceived, have argued that the "real table" depends on being perceived by some mind, whether God's or the collective mind of the universe.

  • Reasons for Believing in a Real Table: Despite the philosophical difficulties, most philosophers agree that there is a "real table" that exists independently of our perception of it. The next chapter will consider the reasons for this belief.

  • Philosophical Questioning: Even the most familiar objects, like a table, become philosophically puzzling when we question the relationship between their appearance and their underlying reality. Philosophy has the power to increase the interest and wonder in the world by asking such questions.


Here are the key takeaways from the chapter:

  • Certainty of Immediate Experiences: The author argues that while we may doubt the physical existence of objects like a table, we cannot doubt the existence of our immediate sense experiences, such as the color and shape we perceive or the sensation of hardness we feel. These immediate experiences are psychologically certain.

  • Descartes' Cogito Argument: Descartes' famous argument "I think, therefore I am" (Cogito, ergo sum) establishes the certainty of one's own existence, since the very act of doubting one's existence implies that one must exist. This was a significant contribution to philosophy.

  • Distinction between Sense-data and the Self: The author notes that Descartes' argument about the certainty of one's own existence may say more than is strictly warranted. The "I" or self is not as immediately certain as particular sense experiences.

  • Sense-data as a Basis for Knowledge: The author argues that our particular thoughts and feelings, our sense-data, provide a solid basis from which to begin the pursuit of knowledge, as they have a primitive certainty.

  • The Problem of the External World: The key problem the author seeks to address is whether we have any reason to believe that our sense-data correspond to the existence of physical objects independent of our perception, or if the external world is merely a product of our imagination.

  • Common Sense Belief in an External World: The author notes that common sense strongly believes in the existence of a physical world independent of our sense-data, as it seems absurd to think that objects would cease to exist when we are not perceiving them.

  • Argument for the External World: The author's main argument for the existence of an external world is that it provides a simpler and more coherent explanation for the regularities and similarities we observe in our sense-data, compared to the hypothesis that the entire external world is merely a product of our imagination.

  • Philosophical Approach to Instinctive Beliefs: The author argues that philosophy should systematically examine our instinctive beliefs, identifying the strongest and most fundamental ones, and showing how they form a harmonious system. This can justify retaining these beliefs, even if they cannot be conclusively proven.


  • Sense-data and Physical Objects: The chapter discusses the distinction between our sense-data (e.g., the color, hardness, and sound of a table) and the physical objects that we believe exist independently of our perceptions. The physical objects are assumed to be the "real" things that our sense-data are signs or appearances of.

  • Scientific View of Matter: According to the scientific view, all natural phenomena can be reduced to motions, either of "aether" or "gross matter." Science assigns only two properties to matter: position in space and the power of motion. Other properties, such as color, sound, and space as we perceive it, are not considered part of the scientific conception of the physical world.

  • Distinction between Perceived Space and Physical Space: The chapter argues that the space we perceive through sight and touch is different from the "physical space" of science. The physical space must be "neutral" between different perceptual spaces, and it is this physical space that is the subject of geometry and physics.

  • Knowledge of Physical Space: We can know the relations and arrangements of objects in physical space, such as their relative positions and the fact that they are in a straight line during an eclipse. However, we cannot have direct acquaintance with the nature of physical space itself, just as a blind person cannot have direct acquaintance with the space of sight.

  • Distinction between Perceived Time and Physical Time: Similar to the distinction between perceived space and physical space, the chapter argues that there is a distinction between our subjective experience of time (duration) and the "public time" that is the subject of physics.

  • Limitations of Sense-data in Representing Physical Objects: The chapter argues that the qualities we perceive in sense-data, such as color, cannot be directly attributed to the physical objects themselves. The physical properties that correspond to our sense-data, such as wave-motions, are not directly accessible to us.

  • Rejection of the Naive Realist View: The chapter rejects the "naive realist" view that physical objects are more or less like our sense-data, with the "real" color of an object being some kind of average of the different colors we perceive. This view is considered groundless, as the color we see depends on the nature of the light waves reaching the eye, not solely on the object itself.

  • Idealist Perspective: The chapter mentions that some philosophers, called "idealists," have argued that what appears as matter is really something mental, either rudimentary minds or ideas in the mind. The chapter states that the reasons for this idealist view will be considered in the next chapter.


  • Idealism: The doctrine that whatever exists or can be known to exist must be in some sense mental. This doctrine has several forms and is advocated on different grounds.

  • Berkeley's Argument: Berkeley argued that our sense-data cannot be supposed to have an independent existence, but must be 'in' the mind. He further argued that to be known is to be 'in' a mind, and therefore, everything that can be known must be mental.

  • Confusion of 'Idea': Berkeley's argument relies on a confusion between the act of apprehension (which is mental) and the thing apprehended (which may or may not be mental). This confusion leads to the fallacious conclusion that whatever can be apprehended must be mental.

  • Distinction between Knowledge by Acquaintance and Knowledge by Description: We can have knowledge of the existence of something without being acquainted with it. In such cases, we know the thing by description, and its existence can be inferred from the existence of something with which we are acquainted.

  • Rejection of the Claim that We Cannot Know Anything that We are not Acquainted With: This claim is false. We can truly judge the existence of things with which we are not acquainted, based on our knowledge of general principles and our acquaintance with other things.


  • Knowledge by Acquaintance vs. Knowledge by Description: The chapter distinguishes between two types of knowledge: knowledge by acquaintance, which is direct awareness of things, and knowledge by description, which is knowledge about things through descriptions.

  • Acquaintance with Sense-Data: We have acquaintance with the sense-data that make up the appearance of objects, such as the color, shape, and texture of a table. This is direct, immediate knowledge without any inference or knowledge of truths.

  • Knowledge of Physical Objects: Our knowledge of physical objects, like the table, is not direct acquaintance but rather "knowledge by description." We know the table through the sense-data it causes, not through direct awareness of the object itself.

  • Acquaintance with Mental Events: In addition to sense-data, we have acquaintance with the contents of our own minds, such as thoughts, feelings, and desires. This self-consciousness is the source of our knowledge of mental events.

  • Acquaintance with Universals: We have acquaintance not only with particular existing things but also with universals or general ideas, such as whiteness, diversity, and brotherhood. Awareness of universals is called "conceiving," and a universal we are aware of is called a "concept."

  • Descriptions and Definite Descriptions: A "description" is any phrase of the form "a so-and-so" or "the so-and-so." A "definite description" is a phrase of the form "the so-and-so," which implies there is a unique object that satisfies the description.

  • Knowledge by Description: We can have knowledge about objects that we are not acquainted with, through "knowledge by description." This involves knowing that there is a unique object satisfying a certain description, even if we are not directly acquainted with that object.

  • Proper Names as Descriptions: Even proper names are often really descriptions, as the thought associated with a name usually involves a description rather than direct acquaintance with the object.

  • Limits of Private Experience: Knowledge by description allows us to go beyond the limits of our private experience, as we can have knowledge of things we have never directly experienced.


  • Induction and Inference: The chapter discusses the problem of induction, which is how we can draw inferences about the existence of things beyond our immediate experience, such as the existence of matter, other people, the past before our memory, and the future. The author argues that we must rely on some general principles or laws to make such inferences.

  • The Belief in the Sun's Sunrise: The author uses the example of our belief that the sun will rise tomorrow as an illustration of the problem of induction. This belief is based on the past experience of the sun rising every day, but the author questions whether this past experience is sufficient justification for the belief in the sun's future rising.

  • The Principle of Induction: The author proposes the "principle of induction" as the general principle that allows us to make inferences from past experience to the future. This principle has two parts:

    • The more often two things have been found associated, the more probable it is that they will be associated in a fresh case.
    • A sufficient number of cases of association will make the probability of a fresh association nearly certain.
  • The Probability of the General Law: The author extends the principle of induction to the probability of a general law, stating that the more cases of association between two things are known, and the fewer cases of dissociation, the more probable it is that the general law of their association is true.

  • Probability and Relativity: The author notes that the probability of an inductive inference is always relative to the available data. New data can change the probability, and the fact that expectations are sometimes disappointed does not disprove the inductive principle.

  • The Justification of Induction: The author argues that the inductive principle cannot be proved by experience, as all arguments from experience to the future or the unexperienced rely on the inductive principle itself. Therefore, the inductive principle must be accepted on the basis of its "intrinsic evidence" or else all our expectations about the future and our conduct based on past experience must be abandoned.

  • The Scope and Certainty of Inductive Knowledge: The author suggests that the problems raised by the justification of inductive knowledge, including the belief in the "reign of law" and the principle of causality, are among the most difficult and debated problems in philosophy, and will be further explored in the next chapter.


  • Logical Principles: There are certain logical principles, such as "anything implied by a true proposition is true", that are self-evident and cannot be proved or disproved by experience. These principles are necessary for drawing inferences from what is given in sensation.

  • Laws of Thought: The three traditional "Laws of Thought" - the law of identity, the law of contradiction, and the law of excluded middle - are examples of self-evident logical principles, but they are not more fundamental or self-evident than other similar principles.

  • A Priori Knowledge: In addition to logical principles, there are other types of a priori knowledge, such as knowledge of ethical values, that are not derived from experience but are elicited and caused by experience.

  • Empiricism vs. Rationalism: The debate between empiricists and rationalists has been resolved, with the recognition that while all knowledge is elicited and caused by experience, some knowledge is a priori in the sense that the experience that makes us aware of it does not suffice to prove it.

  • Limits of A Priori Knowledge: A priori knowledge is strictly limited - it can tell us that if one thing exists, another must exist, or that if one proposition is true, another must be true, but it cannot tell us that anything actually exists in the real world.

  • Deduction vs. Induction: In the case of a priori knowledge, such as mathematical judgments, deduction can provide new knowledge, whereas in the case of empirical generalizations, induction is theoretically preferable and warrants greater confidence in the truth of the conclusion.

  • The Problem of Infinite Instances: The question of how we can have knowledge of general propositions, when we can never examine all the infinite instances, is a difficult and historically important problem first raised by Kant.


  • A Priori Knowledge: Kant recognized that we have a priori knowledge, which is not purely "analytic" (where the predicate is contained within the subject). This challenged the prevailing view that all a priori knowledge was analytic.

  • Synthetic A Priori Knowledge: Kant identified that many a priori propositions, such as those in mathematics and geometry, are "synthetic" - the predicate is not contained within the subject. This raised the question of how such synthetic a priori knowledge is possible.

  • Kant's Solution: Kant's solution was that the a priori forms of intuition (space and time) and the categories of the understanding are contributed by the mind, and thus ensure that our experience will conform to these a priori structures. However, this limits the scope of a priori knowledge to the realm of possible experience.

  • Limitations of Kant's Solution: The main issue with Kant's solution is that it does not account for the certainty and universality of a priori truths. If these truths are merely features of our minds, there is no guarantee they will remain constant or apply to reality beyond our experience.

  • A Priori Knowledge as Knowledge of Things, Not Just Thoughts: The author argues that a priori knowledge, such as the law of contradiction or mathematical truths, is not merely about the constitution of our minds, but about objective facts concerning things in the world, both mental and non-mental.

  • The Realm of Universals: The author suggests that a priori knowledge is concerned with "entities which do not, properly speaking, exist" - such as qualities and relations. These are not mental or physical, but belong to a distinct realm of universals.


Here are the key takeaways from the chapter:

  • Universals vs. Particulars: Universals are entities that are opposed to particular things given in sensation. Universals can be shared by many particulars and have characteristics that distinguish them from particular things, such as being immutable and indestructible.

  • Plato's Theory of Ideas: Plato's theory of ideas is an attempt to solve the problem of the nature of universals. Plato posits a "supra-sensible" world of ideas or forms that are more real than the world of particular things, and which give the world of sense its pale reflection of reality.

  • Language and Universals: Most words in the dictionary, except for proper names and pronouns, stand for universals rather than particulars. This fact is often overlooked, as we tend to focus on the particular things that words refer to rather than the universals they denote.

  • Universals and Relations: The neglect of relations (expressed by verbs and prepositions) in favor of qualities (expressed by adjectives and nouns) has led to philosophical views like monism and monadism, which deny the reality of relations.

  • The Being of Universals: Universals do not exist in the same sense that particular things exist in space and time. Rather, universals "subsist" or have a timeless, non-mental being. This peculiar kind of being has led some to mistakenly view universals as mental entities.

  • The Two Worlds: There are two realms: the world of universals, which is unchangeable and perfect, and the world of particulars, which is fleeting and imperfect. Both realms are important for the metaphysician to consider.

  • Knowledge of Universals: The problem of our knowledge of universals will be the focus of the next chapter, as it is connected to the problem of a priori knowledge.


Here are the key takeaways from the chapter:

  • Knowledge of Universals by Acquaintance: We can be acquainted with certain universals, such as sensible qualities (e.g., white, red, sweet, loud) and relations (e.g., spatial, temporal, and resemblance relations). This acquaintance is gained through abstraction from our experience of particular instances of these universals.

  • Relations between Universals: We can also be immediately aware of relations between universals, such as the relation of "greater than" between two resemblance relations.

  • A Priori Knowledge and Universals: All a priori knowledge deals exclusively with the relations of universals. Propositions like "two and two are four" are not about particular instances, but about the universals "two" and "four" and the relation between them.

  • Difference between A Priori and Empirical Knowledge: The difference between a priori general propositions and empirical generalizations lies not in their meaning, but in the nature of the evidence. A priori propositions are known through the perception of relations between universals, while empirical generalizations are based on particular instances.

  • Knowledge of Universals without Instances: It is possible to have knowledge of general propositions about universals even when no particular instances of those universals can be given. This is because such knowledge only requires knowledge of the relations between the universals, not knowledge of particular instances.

  • Sources of Knowledge: Our knowledge can be divided into knowledge of things (by acquaintance or description) and knowledge of truths (intuitive or derivative). Intuitive knowledge of truths includes self-evident logical, arithmetical, and ethical principles.

  • The Problem of Error: While knowledge by acquaintance is immune to error, knowledge of truths raises the problem of distinguishing knowledge from error, as some of our beliefs may turn out to be erroneous.


  • Intuitive Knowledge: There are two types of intuitive knowledge - general principles (e.g., logical and mathematical principles) and judgements of perception (i.e., self-evident truths derived from sensory experience).

  • Self-Evidence: Self-evidence is not an all-or-nothing quality, but rather a matter of degree. Certain truths, such as those of logic and mathematics, have a very high degree of self-evidence, while others, such as ethical or aesthetic judgements, have a lower degree of self-evidence.

  • Memory: Memories are a form of intuitive knowledge, distinct from the images that may accompany them. While memory can be fallible, recent and vivid memories tend to have a high degree of self-evidence and trustworthiness.

  • Degrees of Self-Evidence: The varying degrees of self-evidence correspond to the varying degrees of trustworthiness. When there is a conflict between propositions, the more self-evident proposition should be retained, and the less self-evident one rejected.

  • Infallible Guarantee of Truth: The author suggests that the highest degree of self-evidence may be an infallible guarantee of truth, while the lower degrees of self-evidence may only provide a greater or lesser presumption of truth, but not an absolute guarantee.


Here are the key takeaways from the chapter:

  • Truth and Falsehood are Properties of Beliefs: Truth and falsehood are properties of beliefs and statements, not of things or acquaintance. A world of mere matter would contain no truth or falsehood, only facts.

  • Truth Depends on the Relation of Beliefs to External Facts: The truth or falsehood of a belief depends on its relation to external facts, not on any internal quality of the belief itself. A belief is true if it corresponds to a fact, and false if it does not.

  • Coherence is an Insufficient Definition of Truth: The theory that truth consists in the coherence of a belief within a system of beliefs is problematic, as there can be multiple coherent systems of beliefs, and the laws of logic that define coherence cannot themselves be established by the coherence test.

  • Beliefs Involve a Relation of the Mind to Multiple Terms: Beliefs cannot be simply a relation of the mind to a single object, as this would not allow for the possibility of false beliefs. Beliefs must involve a relation of the mind to multiple terms, including the subject, objects, and the relation believed.

  • Truth Requires a Corresponding Complex Fact: For a belief to be true, there must be a corresponding complex fact composed of the same constituents (subject, objects, and relation) in the same order as in the belief. If there is no such corresponding fact, the belief is false.

  • Minds Create Beliefs but Do Not Create Truth or Falsehood: Minds create beliefs, but the truth or falsehood of a belief depends on its correspondence to external facts, not on the mind itself. Minds cannot make a belief true or false, except in cases where the belief concerns the future actions of the believer.


Here are the key takeaways from the chapter:

  • True Belief vs. Knowledge: A true belief does not necessarily constitute knowledge. For example, if a person believes that the late Prime Minister's last name began with a B, and this is true, it does not mean the person has knowledge of this fact. Knowledge requires more than just a true belief.

  • Derivative Knowledge: Derivative knowledge is what is validly deduced from premisses known intuitively. However, this definition is problematic as it assumes the existence of "known premisses" which needs to be defined.

  • Psychological Inference: People often arrive at beliefs not through logical inference, but through psychological processes like association. As long as there is a valid logical connection that could be discovered, these beliefs can be considered derivative knowledge.

  • Intuitive Knowledge: Intuitive knowledge, as opposed to derivative knowledge, is more difficult to define. There is no clear criterion to distinguish true intuitive beliefs from erroneous ones.

  • Self-Evidence: There are two types of self-evidence: (1) Absolute self-evidence, which occurs when one has direct acquaintance with a fact, and (2) Partial self-evidence, which occurs in judgments and has varying degrees of certainty.

  • Degrees of Certainty: Knowledge, error, and probable opinion exist on a spectrum. Beliefs can range from absolutely certain intuitive knowledge to mere probable opinions with low degrees of self-evidence.

  • Coherence: While coherence cannot define truth, it can be used as a criterion to increase the probability of a set of beliefs. Mutually coherent probable opinions become more probable than any one of them in isolation.

  • Limits of Probable Opinion: Even a highly coherent system of probable opinions cannot be transformed into absolute, indubitable knowledge through organization alone. Some degree of certainty must already exist at some point in the system.


Here are the key takeaways from the chapter:

  • Limits of Metaphysical Reasoning: The chapter argues that the ambitious metaphysical claims made by many philosophers, such as proving the fundamental dogmas of religion or the essential rationality of the universe, are not valid. The proposed a priori proofs of such theses are not capable of surviving critical scrutiny.

  • Hegel's Absolute Idealism: The chapter examines Hegel's philosophy as a representative of the kind of metaphysical reasoning it aims to critique. Hegel's main thesis is that every part of reality is incomplete and requires the rest of the universe to exist, leading him to conclude that Absolute Reality is a single, harmonious, spiritual system.

  • Critique of Hegel's Argument: The chapter argues that Hegel's fundamental tenet, that what is incomplete cannot be self-subsistent, is flawed. It points out that a truth about a thing is not part of the thing itself, and that the mere fact that a thing has relations does not prove that those relations are logically necessary.

  • Limits of A Priori Reasoning: The chapter concludes that we cannot prove that the universe as a whole forms a single harmonious system, as Hegel believes. It also argues that we cannot prove the unreality of space, time, matter, and evil through a priori reasoning alone.

  • Piecemeal Investigation of the World: Given the limitations of metaphysical reasoning, the chapter suggests that we are left to the piecemeal investigation of the world through empirical and scientific methods, rather than being able to know the characters of parts of the universe remote from our experience.

  • Shift in Modern Thought: The chapter notes that the tendency of modern thought is increasingly towards showing that the supposed contradictions in our understanding of the world are illusory, and that very little can be proved a priori about the nature of reality.

  • Expanded Possibilities in Mathematics: The chapter uses the example of space and time to illustrate how mathematics has expanded our understanding of possible forms of space, undermining the previous philosophical arguments about the necessity of Euclidean geometry.

  • Philosophical Knowledge and Scientific Knowledge: The chapter argues that philosophical knowledge does not differ essentially from scientific knowledge, as both rely on a combination of a priori principles and empirical knowledge. The distinctive feature of philosophy is its critical examination of the principles employed in science and daily life.

  • Limits of Philosophical Criticism: The chapter acknowledges that philosophical criticism cannot be taken to an extreme of complete skepticism, as some basic knowledge is indubitable. The aim of philosophical criticism is to retain whatever still appears to be knowledge after close examination, not to reject beliefs without reason.


Here are the key takeaways from the chapter:

  • The Value of Philosophy Lies in its Effects, Not its Utility: The value of philosophy does not lie in its direct utility or ability to provide definitive answers, but rather in its indirect effects on the lives of those who study it.

  • Philosophy Aims at Knowledge and Critical Examination: Philosophy primarily aims at the kind of knowledge that provides unity and system to the sciences, and the kind that results from a critical examination of the grounds of our convictions, prejudices, and beliefs.

  • Philosophy Grapples with Fundamental, Unanswerable Questions: Philosophy grapples with profound questions about the nature of the universe, consciousness, and the significance of good and evil, which may not have demonstrably true answers.

  • Philosophy Frees the Mind from Prejudices and Expands Possibilities: By questioning the taken-for-granted assumptions of common sense and habitual beliefs, philosophy frees the mind from the "tyranny of custom" and opens it up to new and unfamiliar possibilities.

  • Philosophy Promotes Impartial, Contemplative Thought: Philosophic contemplation adopts an impartial, dispassionate stance towards the world, seeking knowledge for its own sake rather than for personal or practical ends. This promotes a "greatness of soul" and a sense of union with the vastness of the universe.

  • Philosophy Enlarges the Self and Fosters True Freedom: By expanding the mind's conception of what is possible and encouraging an impartial, contemplative attitude, philosophy enables the individual to transcend the limitations of their private, instinctive interests and achieve a true freedom and citizenship in the universe.


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