10 2.3 Rationalist and Empiricists – Continued

In this section, we meet a noted empiricist who casts doubt on the very possibility of acquiring knowledge of the world. This new wrinkle in empiricist speculation inspires a creative rebuttal based on the interactive roles of experience and reason.

2.3.1 Hume: Empiricism and Doubt

David Hume (1711-1776) was a Scottish philosopher whose work was not overwhelmingly well received in his lifetime but had major impact later on empiricism and on philosophy of science. His 1748 work An Enquiry Concerning Human Understanding provided a more accessible account of his empiricism as originally published.

Note: Portions of the following material on Hume are adapted from information in The Philosophy Pages by Garth Kemerling and which is licensed under a Creative Commons Attribution-ShareAlike 3.0

Hume’s position is that since human beings do in fact live and function in the world, we should try to observe how they do so. The key principle to be applied to any investigation of our cognitive capacities is an attempt to discover the causes of human belief. This attempt is neither the popular project of noticing and cataloging human beliefs nor the metaphysical effort to provide them with an infallible rational justification. According to Hume, the proper goal of philosophy is simply to explain why we believe what we do.


Hume’s analysis of human belief begins with a careful distinction between certain mental contents:

  • Impressions are the direct, vivid, and forceful products of immediate experience.
  • Ideas are merely feeble copies of these original impressions.

From Section II of An Enquiry:

Every one will readily allow, that there is a considerable difference between the perceptions of the mind, when a man feels the pain of excessive heat, or the pleasure of moderate warmth, and when he afterwards recalls to his memory this sensation, or anticipates it by his imagination. These faculties may mimic or copy the perceptions of the senses; but they never can entirely reach the force and vivacity of the original sentiment. The utmost we say of them, even when they operate with greatest vigour, is, that they represent their object in so lively a manner, that we could almost say we feel or see it: but, except the mind be disordered by disease or madness, they never can arrive at such a pitch of vivacity, as to render these perceptions altogether undistinguishable. All the colours of poetry, however splendid, can never paint natural objects in such a manner as to make the description be taken for a real landscape. The most lively thought is still inferior to the dullest sensation.

We may observe a like distinction to run through all the other perceptions of the mind. A man in a fit of anger, is actuated in a very different manner from one who only thinks of that emotion. If you tell me, that any person is in love, I easily understand your meaning, and form a just conception of his situation; but never can mistake that conception for the real disorders and agitations of the passion. When we reflect on our past sentiments and affections, our thought is a faithful mirror, and copies its objects truly; but the colours which it employs are faint and dull, in comparison of those in which our original perceptions were clothed. It requires no nice discernment or metaphysical head to mark the distinction between them.

Here therefore we may divide all the perceptions of the mind into two classes or species, which are distinguished by their different degrees of force and vivacity. The less forcible and lively are commonly denominated Thoughts or Ideas. The other species want a name in our language, and in most others; I suppose, because it was not requisite for any, but philosophical purposes, to rank them under a general term or appellation. Let us, therefore, use a little freedom, and call them Impressions; employing that word in a sense somewhat different from the usual. By the term impression, then, I mean all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will. And impressions are distinguished from ideas, which are the less lively perceptions, of which we are conscious, when we reflect on any of those sensations or movements above mentioned.

The background color of the screen at which you are now looking is an impression, while your memory of the color of your first dog (if you’ve had dogs) is merely an idea. Since every idea must be derived from an antecedent impression, Hume supposed, it always makes sense to inquire into the origins of our ideas by asking from which impressions they are derived.

Add to this that each of our ideas and impressions is entirely separable from every other, in Hume’s view. The apparent connection of one idea to another is invariably the result of an association that we manufacture ourselves.

From Section III of Enquiry:

Though it be too obvious to escape observation, that different ideas are connected together; I do not find that any philosopher has attempted to enumerate or class all the principles of association; a subject, however, that seems worthy of curiosity. To me, there appear to be only three principles of connexion among ideas, namely, Resemblance, Contiguity in time or place, and Cause or Effect.

That these principles serve to connect ideas will not, I believe, be much doubted. A picture naturally leads our thoughts to the original: the mention of one apartment in a building naturally introduces an enquiry or discourse concerning the others: and if we think of a wound, we can scarcely forbear reflecting on the pain which follows it. But that this enumeration is complete, and that there are no other principles of association except these, may be difficult to prove to the satisfaction of the reader, or even to a man’s own satisfaction. All we can do, in such cases, is to run over several instances, and examine carefully the principle which binds the different thoughts to each other, never stopping till we render the principle as general as possible. The more instances we examine, and the more care we employ, the more assurance shall we acquire, that the enumeration, which we form from the whole, is complete and entire.

Experience provides us with both the ideas themselves and our awareness of their association. All human beliefs (including those we regard as cases of knowledge) result from repeated applications of these simple associations.

In Section IV of Enquiry, Hume further distinguished between two sorts of belief:

  • Relations of ideas are beliefs grounded wholly on associations formed within the mind; they are capable of demonstration because they have no external referent.
  • Matters of fact are beliefs that claim to report the nature of existing things; they are always contingent.

These distinctions are Hume’s version of the a priori versus a posteriori distinction. Mathematical and logical knowledge relies upon relations of ideas; it is uncontroversial but uninformative with respect to knowledge the world. The interesting but problematic propositions of natural science depend upon matters of fact. Abstract metaphysics mistakenly (and fruitlessly) tries to achieve the certainty of the former with the content of the latter.

Matters of Fact and Skepticism

Since genuine information rests upon our belief in matters of fact, Hume was particularly concerned to explain their origin. Such beliefs can reach beyond the content of present sense-impressions and memory, Hume held, only by appealing to presumed connections of cause and effect. But since each idea is distinct and separable from every other, there is no self-evident relation; these connections can only be derived from our experience of similar cases. So the crucial question in epistemology is to ask exactly how it is possible for us to learn from experience.

From Enquiry, Section IV, Part 1:

All reasonings concerning matter of fact seem to be founded on the relation of Cause and Effect. By means of that relation alone we can go beyond the evidence of our memory and senses. If you were to ask a man, why he believes any matter of fact, which is absent; for instance, that his friend is in the country, or in France; he would give you a reason; and this reason would be some other fact; as a letter received from him, or the knowledge of his former resolutions and promises. A man finding a watch or any other machine in a desert island, would conclude that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious. The hearing of an articulate voice and rational discourse in the dark assures us of the presence of some person: Why? because these are the effects of the human make and fabric, and closely connected with it. If we anatomize all the other reasonings of this nature, we shall find that they are founded on the relation of cause and effect, and that this relation is either near or remote, direct or collateral. Heat and light are collateral effects of fire, and the one effect may justly be inferred from the other.

If we would satisfy ourselves, therefore, concerning the nature of that evidence, which assures us of matters of fact, we must enquire how we arrive at the knowledge of cause and effect.

I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other. Let an object be presented to a man of ever so strong natural reason and abilities; if that object be entirely new to him, he will not be able, by the most accurate examination of its sensible qualities, to discover any of its causes or effects. Adam, though his rational faculties be supposed, at the very first, entirely perfect, could not have inferred from the fluidity and transparency of water that it would suffocate him, or from the light and warmth of fire that it would consume him. No object ever discovers, by the qualities which appear to the senses, either the causes which produced it, or the effects which will arise from it; nor can our reason, unassisted by experience, ever draw any inference concerning real existence and matter of fact.

This proposition, that causes and effects are discoverable, not by reason but by experience, will readily be admitted with regard to such objects, as we remember to have once been altogether unknown to us; since we must be conscious of the utter inability, which we then lay under, of foretelling what would arise from them. Present two smooth pieces of marble to a man who has no tincture of natural philosophy; he will never discover that they will adhere together in such a manner as to require great force to separate them in a direct line, while they make so small a resistance to a lateral pressure. Such events, as bear little analogy to the common course of nature, are also readily confessed to be known only by experience; nor does any man imagine that the explosion of gunpowder, or the attraction of a loadstone, could ever be discovered by arguments a priori. In like manner, when an effect is supposed to depend upon an intricate machinery or secret structure of parts, we make no difficulty in attributing all our knowledge of it to experience. Who will assert that he can give the ultimate reason, why milk or bread is proper nourishment for a man, not for a lion or a tiger?

Here, Hume supposed, the most obvious point is a negative one: causal reasoning can never be justified rationally. In order to learn, we must suppose that our past experiences bear some relevance to present and future cases. But although we do indeed believe that the future will be like the past, the truth of that belief is not self-evident. In fact, it is always possible for nature to change, so inferences from past to future are never rationally certain. Thus, in Hume’s view, the principle of induction cannot lead to meaningful conclusions about the world, and all beliefs in matters of fact are fundamentally non-rational.

…we always presume, when we see like sensible qualities, that they have like secret powers, and expect that effects, similar to those which we have experienced, will follow from them. If a body of like colour and consistence with that bread, which we have formerly eat, be presented to us, we make no scruple of repeating the experiment, and foresee, with certainty, like nourishment and support. Now this is a process of the mind or thought, of which I would willingly know the foundation. It is allowed on all hands that there is no known connexion between the sensible qualities and the secret powers; and consequently, that the mind is not led to form such a conclusion concerning their constant and regular conjunction, by anything which it knows of their nature. As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, nourished me; that is, a body of such sensible qualities was, at that time, endued with such secret powers: but does it follow, that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary. At least, it must be acknowledged that there is here a consequence drawn by the mind; that there is a certain step taken; a process of thought, and an inference, which wants to be explained. These two propositions are far from being the same. I have found that such an object has always been attended with such an effect, and I foresee, that other objects, which are, in appearance, similar, will be attended with similar effects. I shall allow, if you please, that the one proposition may justly be inferred from the other: I know, in fact, that it always is inferred. But if you insist that the inference is made by a chain of reasoning, I desire you to produce that reasoning. The connexion between these propositions is not intuitive. There is required a medium, which may enable the mind to draw such an inference, if indeed it be drawn by reasoning and argument. What that medium is, I must confess, passes my comprehension; and it is incumbent on those to produce it, who assert that it really exists, and is the origin of all our conclusions concerning matter of fact.

Consider Hume’s favorite example: our belief that the sun will rise tomorrow. Clearly, this is a matter of fact; it rests on our conviction that each sunrise is an effect caused by the rotation of the earth. But our belief in that causal relation is based on past observations, and our confidence that it will continue tomorrow cannot be justified inductively by reference to the past. So we have no rational basis for believing that the sun will rise tomorrow. Yet we do believe it!

Mitigated Skepticism

Where does this leave us? Hume believed he was carrying out the empiricist program with rigorous consistency. Locke honestly proposed the possibility of deriving knowledge from experience, but did not carry it far enough. Hume demonstrates that empiricism inevitably leads to an utter and total skepticism.

According to Hume, knowledge of pure mathematics is secure because it rests only on the relations of ideas, without presuming anything about the world. Experimental observations (conducted without any assumption of the existence of material objects) permit us to use our experience in forming useful habits. Any other epistemological effort, especially if it involves the pretense of achieving useful abstract knowledge, is meaningless and unreliable.

The most reasonable position, Hume held, is a “mitigated” skepticism that humbly accepts the limitations of human knowledge while pursuing the legitimate aims of math and science. In our non-philosophical moments, of course, we will be thrown back upon the natural beliefs of everyday life, no matter how lacking in rational justification we know they are.

Hume Summary

David Hume was an empiricist who doubted the principle of cause and effect, the principle of induction, and the possibility of actually knowing an external world. According to Hume, “…every effect is a distinct event from its cause.”

  • We cannot know a priori that such a connection exists between any two events, because, if we were witnessing a supposed causal connection for the first time, simply using reason could not lead us to know that we were seeing cause and effect. We might have witnessed a random occurrence or correlation.
  • We cannot know a posteriori that there is a causal connection between any two events, because there is nothing in our direct observation of events that denotes that one is a cause and the other an effect.

Hume maintained that inferences from past to future are never rationally certain, and thus, the principle of induction cannot lead to meaningful conclusions about the world. Neither a priori activity of the mind (ideas and the relations of ideas that we come to believe) nor a posteriori experience (impressions and the matters-of-fact that we come to believe) can suggest or validate the existence of the external world.

Supplemental resources are available (bottom of page) on Hume’s skepticism.


Briefly explain Hume’s skepticism. Do you think he makes a good argument for his position of doubt? (100 – 150 words)

Note: Submit your response to the appropriate Assignments folder.

2.3.2 Kant: A Reasoned Response to Skepticism

Immanuel Kant (1724-1804), an innovative philosopher born in East Prussia (now Germany), appeared on the scene at a time of disarray in the world of Western epistemological thought. Rationalists and empiricists were at serious odds with each other. Pure rationalism did not offer experience a valued place in acquisition of true knowledge. The possibility of acquiring certain knowledge through experience, as we have just seen in our material on David Hume, was in a crisis of skepticism and doubt.

As mentioned previously, asking epistemological questions can entail additional questions about metaphysics; a theory that explains how we acquire knowledge is deeply intertwined with a theory on what is actually “out there” to be known. Kant creates a complex but compelling theory of knowledge known as Transcendental Idealism, which describes truths about the world as both necessary and universal. Kant first published his vast masterwork of epistemology, the Critique of Pure Reason in 1781 and revised it in 1787. Between editions of the Critique, in 1783 he published the Prolegomena to any Future Metaphysic, in which he presented topics from the Critiquein a manner that serves as an introduction to it. The Critique is regarded by some, (even by Kant!) as intricate and perplexing. Our examination here of Kant and Transcendental Idealism will refer to both works.

Note: Portions of the following material on Kant are adapted from information in The Philosophy Pages by Garth Kemerling and is licensed under a Creative Commons Attribution-ShareAlike 3.0

Kant’s aim was to move beyond the traditional dichotomy between rationalism and empiricism:

  • The rationalists had tried to show that we can understand the world by careful use of reason. This guarantees undoubtable knowledge but leaves serious questions about its practical content.
  • The empiricists had argued that all of our knowledge must be firmly grounded in experience. Practical content is thus secured, but it turns out that we can be certain of very little.

Kant surmised that both approaches failed because they are premised on similar mistaken assumptions.

Progress in philosophy, according to Kant, requires that we frame the epistemological problem differently:

  • The crucial question is how the world comes to be understood by us, not how we can bring ourselves to understand the world.
  • We must allow the structure of our concepts to shape our experience of objects, instead of trying, by reason or experience, to make our concepts match the nature of objects.
  • We must see our minds as actively interacting with the products of experience, not as passive receivers of perceptions.

The purpose of Kant’s Critique of Pure Reason is to show how reason determines the conditions under which experience and knowledge are possible. The Critique’s Introduction: begins as follows:

I. Of the difference between Pure and Empirical Knowledge

That all our knowledge begins with experience there can be no doubt. For how is it possible that the faculty of cognition should be awakened into exercise otherwise than by means of objects which affect our senses, and partly of themselves produce representations, partly rouse our powers of understanding into activity, to compare to connect, or to separate these, and so to convert the raw material of our sensuous impressions into a knowledge of objects, which is called experience? In respect of time, therefore, no knowledge of ours is antecedent to experience, but begins with it.

But, though all our knowledge begins with experience, it by no means follows that all arises out of experience. For, on the contrary, it is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself (sensuous impressions giving merely the occasion), an addition which we cannot distinguish from the original element given by sense, till long practice has made us attentive to, and skillful in separating it. It is, therefore, a question which requires close investigation, and not to be answered at first sight, whether there exists a knowledge altogether independent of experience, and even of all sensuous impressions? Knowledge of this kind is called a priori, in contradistinction to empirical knowledge, which has its sources a posteriori, that is, in experience.

In the Prolegomena to Any Future Metaphysic (1783) Kant presented the central themes of the first Critique in a slightly different manner, starting from instances in which it appears we have achieved knowledge, and then asking: under what conditions does each case become possible? He began by carefully drawing a pair of crucial distinctions among the judgments we actually make:

The first distinction separates a priori from a posteriori judgments by reference to the origin of our knowledge of them.

  • A priori judgments are based upon reason alone, independently of all sensory experience, and therefore apply with strict universality.
  • A posteriori judgments must be grounded upon experience and are consequently limited and uncertain in the the scope of their applicability.

This distinction marks the difference between necessary and contingent truths.

Second is the distinction between analytic and synthetic judgments, according to the information conveyed as their content.

  • Analytic judgments are those whose predicates are wholly contained in their subjects. Such judgments simply explicate the subject, making it plain and clear but adding nothing to its concept.
  • Synthetic judgments are those whose predicates are wholly distinct from their subjects. Such a judgment adds a connection external to the subject’s concept. Synthetic judgments are genuinely informative but require justification by reference to some outside principle.

Kant supposed that previous philosophers had failed to differentiate properly among the possible options available, given these two sets of distinctions. Both Leibniz and Hume had made a single distinction, between:

  • matters-of-fact based on sensory experience, and
  • the uninformative necessary truths of pure reason.

Kant thought these inadequate and limiting. All four of the logically possible combinations should be considered:

  1. Analytic a posteriori judgments cannot arise, since there is never any need to appeal to experience in support of an assertion that simply makes its subject plain and clear.
  2. Synthetic a posteriori judgments are the relatively uncontroversial matters of fact we come to know by means of our sensory experience.
  3. Analytic a priori judgments, everyone agrees, include all merely logical truths and straightforward matters of definition; they are necessarily true.
  4. Synthetic a priori judgments are the crucial case, since only they could provide new information that is necessarily true. Neither Leibniz nor Hume considered the possibility of any such case.

Unlike his predecessors, Kant maintained that synthetic a priori judgments not only are possible but actually provide the basis for significant portions of human knowledge. In fact, he supposed that arithmetic and geometry comprise such judgments and that natural science depends on them for its power to explain and predict events.


Consider, for example, our knowledge that two plus three equals five or that the interior angles of any triangle add up to a straight line (180 degrees).

  • Kant held that these (and other similar) truths of mathematics and geometry are synthetic judgments, since they significantly contribute (add) to our knowledge of the world. The sum of the interior angles is not contained in the concept of a triangle.
  • Yet, clearly, such truths are known a priori, since they apply with strict and universal necessity to all of the objects of our experience, without having been derived from that experience itself.

In these instances, Kant supposed, no one will ask whether or not we have synthetic a priori knowledge; plainly, we do. The question is, how do we come to have such knowledge? If experience does not supply the required connection between the concepts involved, what does?

Kant’s answer is that we do it ourselves!

Conformity with the truths of mathematics is a precondition that we impose upon every possible object of our experience. In order to be perceived by us, any object must be regarded as being uniquely located in space and time, so it is the temporal-spatial framework itself that provides the missing connection between the concept of the triangle and that of the sum of its angles.

Space and time, Kant argued, are the “pure forms of sensible intuition” under which we perceive what we do.

Understanding mathematics in this way makes it possible to rise above an old controversy between rationalists and empiricists regarding the very nature of space and time.

  • Leibniz had maintained that space and time are not intrinsic features of the world itself, but merely a product of our minds.
  • Newton, on the other hand, had insisted that space and time are absolute, not merely a set of spatial and temporal relations.

Kant now declares that both of them were correct! Space and time are absolute, and they do derive from our minds. As synthetic a priori judgments, the truths of mathematics are both informative and necessary.

This is a transcendental deduction, Kant’s method of reasoning that a priori concepts apply correctly/logically to knowledge of the particular. But there is a price to be paid for the certainty we achieve in this manner. Since mathematics derives from our own sensible intuition, we can be absolutely sure that it must apply to everything we perceive. But for the same reason, that it applies from our own sensible intuition, we can have no assurance that it has anything to do with the way things are apart from our own perception of them.

Note: Kant’s use of the term “intuition” refers to a bit of sensory awareness, including any called up by the memory.

Natural Science

No less than in mathematics, in natural science Kant held that synthetic a priorijudgments provide the necessary foundations for human knowledge. The most general laws of nature, like the truths of mathematics, cannot be justified by experience, yet must apply to it universally.

  • Hume’s conclusive demonstration — matters-of-fact rest upon an unjustifiable belief about necessary connection between causes and their effects — seems correct.
  • But Kant’s more constructive approach is to offer a transcendental argument from the fact that we do have knowledge of the natural world to the truth of synthetic a priori propositions about the structure of our experience of it.

As we saw with mathematics, applying the concepts of space and time as forms of sensible intuition is a necessary condition for any perception. But the possibility of scientific knowledge requires that our experience of the world be not only perceivable but thinkable as well, and Kant held that the general intelligibility of experience entails the satisfaction of two further conditions:

  • First, it must be possible in principle to arrange and organize the chaos of our many individual sensory images by tracing the connections that hold among them. Kant called this the “synthetic unity of the sensory manifold.”
  • Second, it must be possible in principle for a single subject to perform this organization by discovering the connections among perceived images. This is satisfied by what Kant called the “transcendental unity of apperception.”

Experiential knowledge is thinkable only if there is some regularity in what is known and there is some knower in whom that regularity can be represented. Since we do actually have knowledge of the world as we experience it, Kant held, both of these conditions are the case.

Deduction of the Categories

Since individual images are perfectly separable as they occur within the sensory manifold, connections between them can be drawn only by the knowing subject in which the principles of connection are to be found. As in mathematics, so in science, the synthetic a priori judgments must derive from the structure of the understanding itself.

Consider the sorts of judgments distinguished by logicians (in Kant’s day). Each of these judgments has:

  • a quantity: universal, particular, singular
  • a quality: affirmative, negative, or infinite
  • a relation: categorical, hypothetical, or disjunctive
  • a modality: possible, actual, or necessary

Kant supposed that any intelligible thought can be expressed in judgments such as these. It follows that any thinkable experience must be understood in these ways, and that we are justified in projecting this entire way of thinking outside ourselves, as the inevitable structure of any possible experience.

The result of Kant’s “transcendental logic” is his schematized table of Transcendental Concepts of the Understanding. These are the concepts, or categories, of understanding used when thinking about the world. Each category is the subject of a separate section of the Critique.

Kant’s Transcendental Concepts
Quantity Quality Relation Modality
Unity Reality Substance Possibility
Plurality Negation Cause Existence
Totality Limitation Community Necessity


Our most fundamental convictions about the natural world derive from these concepts, according to Kant. The most general principles of natural science are not empirical generalizations from what we have experienced. Rather they are synthetic a priori judgments about what we could experience, judgments in which these concepts provide the crucial connectives.

Kant Summary

Kant believed that the external world exists and that gaining knowledge of it is possible using both information from the senses and rational abilities. He reasoned that our minds actively interact with the products of experience, instead of passively receiving perceptions. The structure of our concepts shapes our experience of objects; we make sense of the perceptions that bombard us. We come to know principles such as cause and effect and induction by making the connections between relevant concepts of our understanding and our experiences of the world, for example, that a particular effect follows a particular causative event by necessity. Such truths are both necessary and universal; they are synthetic a priori judgments that provide new information about the world.

Kant’s transcendental idealism maintains that synthetic a priori judgments are possible and provide the basis for significant portions of human knowledge by connecting categories (concepts) of our understanding to our experiences. Kant is not a traditional empiricist because he rejects the notion of the mind as a blank slate, until inscribed by experience, nor is Kant a traditional rationalist, because he does not accept the possibility of a priori ideas that are independent of experience of the world.


Explain how an active-versus-passive role of the human mind contributes to Kant’s position that the external world is knowable? (100 – 200 words)

Note: Submit your response to the appropriate Assignments folder.

Complete the Unit Test by the date on the Schedule of Work.

Supplemental Resources


These short videos on Hume’s skepticism review material provided in the content. [The second video may a queue up automatically when the first is complete.]
• PHILOSOPHY: Epistemology: Hume’s Skepticism and Induction, Part 1
• PHILOSOPHY: Epistemology: Hume’s Skepticism and Induction, Part 2


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