Humes thoughts on metaphysics, more specifically causality, had a major impact on Kant. Although Kant disagreed with many aspects of Humes account, by writing a whole discourse devoted to it, it is obvious that it influenced him greatly. The main disagreement was, for Hume causality was analytic and for Kant it was synthetic. In the Prolegomena, Kant tries to solve some of Humes errors, and at the same time remove his skepticism.
In Book I, Part III, 1 of the Treatise, Hume distinguishes two groups of philosophical relations: those which depend entirely on the ideas, which we compare together, and such as may be changed without any change in the ideas, or Relations of Ideas; and those which dont depend on the ideas alone, or Matters of Fact (Treatise 69). Contiguity, identity, and cause and effect belong to the Matters of Fact. If I consider the idea of my pillow and the idea of my bed, and nothing else, I cant tell whether my pillow is close to my bed.
I may have two resembling perceptions of a tomato, but I cant determine whether Im dealing with the same tomato from the resemblance, no matter how exact. Causation, like contiguity and identity, doesnt depend on the ideas alone; we cant discover causes merely from their idea. But theres a difference between cause and effect and the other two relations which dont depend merely on our ideas, Hume claims. He asserts that these other relations cant be discovered by a mere comparison of ideas, for causation, memory and experience are also needed.
Likewise, in the Enquiry, Hume argues against the a priori justification of causal judgments on the grounds that it is impossible to foretell how objects have behaved prior to having experienced them. One would have to invent an effect, and such invention would be arbitrary. The mind can never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and can never be discovered in it . . . And as the first imagination or invention of a particular effect, in all natural operations, is arbitrary, where we consult not experience; so must we also esteem the supposed tie or connexion between cause and effect, which binds them together, and renders it impossible that any other effect could result from the operation of that cause (Enquiry 18). In the preceding quote, Hume is saying that if reason produces an idea, then those instances of which we have no experience must resemble those of which we do.
From the mere fact that we can imagine an alternate effect, makes the alternate effect a possible and feasible idea. This alone is enough to invalidate the assumption that all future instances of a cause will result in the same effect of all prior instances experienced. Next, Hume attacks the a posteriori justification of causal inference, or the assumption of an effect from a cause based on observation or experience. This attack involves two arguments, which are directed against the universality and the necessity of judgments concerning causal connections.
The first argument is the attack on the justification of induction. Hume says we cannot make an inference from the fact that a certain pattern of succession has always been observed, to the conclusion that the pattern holds in all cases, including those which are unobserved. To make such an inference requires the assumption of the uniformity of nature, and such an assumption is a generalization which cant be justified. It is impossible . . . . that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded upon the supposition of that resemblance.
Let the course of things be allowed hitherto ever so regular; that alone, without some new argument or inference, proves not that, for the future, it will continue to do so (Enquiry 24). There are two theories behind the knowledge from reason or understanding argument; it can either be through intuition/demonstration, or probability. The intuition/demonstration theory fails because we can have an idea of the future being different from the past. Hume says that if we can have an idea of something, then it is possible.
The probability theory also fails because it says we infer any event is true because of experience. However, this relies on the original principle, that the future/present resembles the past. So it is a circular argument. The second argument is directed against necessity. Hume claims that the only necessary connections of which human beings have any knowledge are those found in mathematics. In all Matters of Fact, the only connection we can find is made in the imagination.
After repeated exposure to a pattern of succession, we expect the customary pattern to repeat itself yet again. There arises a feeling of connection in the mind, which is the origin of the idea of a necessary connection. [T]here is nothing is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar; except only, that after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist . . . This connexion, therefore, which we feel in the mind, this customary transition of the imagination from one object to its usual attendant, is the sentiment or impression, from which we form the idea of power or necessary connexion (Enquiry 50). Hume argues that one can never observe in a single instance what event will follow what other event, given that one has never experienced the first before. This can be seen experimentally, in cases in which a person is confronted with something entirely new to him.
After experiencing a repeated pattern of succession of kinds of events, we predict that the pattern will repeat itself at every future time. But there is no difference, in our perception of the events themselves, between the first observation and any of the later ones, except that they have repeated themselves. We can find nothing in the events which can be called cause, power, force, or the like. So our expectation must have its origin in a customary transition in the mind. So for Hume, we have experience of similar objects being conjoined with other similar objects.
He therefore defines cause as, an object, followed by another, and where all objects similar to the first, are followed by objects similar to the second. However, we also have experiences of a cause engendering on the mind an effect, as a result of customary transition. Therefore we may also define cause as an object followed by another, and whose appearance always conveys the thought to that other (Enquiry 51). To Kant, this is equivalent to making causality an illusion. In the Prolegomena, Kant describes the concept of cause, in Humes book, as a bastard of the imagination, impregnated by experience (Prolegomena Introduction).
He finds the consequences of Humes arguments to be altogether destructive of metaphysics, and accuses Hume of overlooking the positive injury which results if reason be deprived of its most important prospects (Prolegomena Introduction). Kant feels that though a great thinker, Hume was hasty and mistaken in his argument (Prolegomena Introduction). Kant gives credit to Hume for starting a well though out, well investigated project. Although he does not agree with Humes conclusions, Kant does feel that he was an acute man, to whom we owe the first spark of light (Prolegomena Introduction).
Hume and Kant both agree that the concept of cause was right, useful, and even indispensable for our knowledge of nature . . . . [the problem that arose was] whether that concept could be though by reason a priori, and consequently whether is possessed an inner truth, independent of all experience, implying a wider application than merely to the objects of experience. Kant feels this is Humes problem. The difference between the two is their concepts concerning the origin of causality. Hume believes that causality, as well as mathematics are analytical.
Alternatively, Kant perceives that not only the connection of cause and effect, but all the propositions of arithmetic and geometry, are synthetic. In all these propositions, no analysis of the subject will reveal the predicate. He gives the example 7+5=12. He points out that 7 and 5 have to be put together to give 12; the idea of 12 is not contained in them, nor even in the idea of adding them together (Prolegomena 2). From his proposition that all mathematics are synthetical, Kant explains how the ideas of space and time are also a priori forms of sensibility.
Geometry is based upon the pure intuition of space. Arithmetic accomplishes its concept of number by the successive addition of units in time; and pure mechanics especially cannot attain its concepts of motion without employing the representation of time. Both representations, however, are only intuitions; for if we omit from the empirical intuitions of bodies and their alterations (motion) everything empirical, or belonging to sensation, space and time still remain, which are therefore pure intuitions that lie a priori at the basis of the empirical (Prolegomena 10).
Kant speculates about how Humes thinking would have been different had he recognized that mathematical judgments are not only a priori but synthetic. The good company into which metaphysics would thus have been brought would have saved it from the danger of a contemptuous ill-treatment, for the thrust intended for it must have reached mathematics, which was not and could not have been Humes intention. Thus that acute man would have been led into considerations which must needs be similar to those that now occupy us, but which would have gained inestimably by his inimtably elegant style (Prolegomena 4).
For Kant, in order to develop experience, we must first make a priori synthesis which yield concepts of the understanding, including causality. It is this very type of synthesis, necessary for the cognition of sensations, that makes experience possible. Causality is valid because of the minds organization of intuitions according to necessary laws, under which the connection is objectively valid and thus becomes experience (Prolegomena 20). Kants solution applies only to perceptions of the phenomenal realm.
He maintains that although one cannot form rationally the concept of one thing causing another, I therefore easily comprehend the concept of cause as a concept necessarily belonging to the mere form of experience, and its possibility as a synthetic unification of perceptions in a consciousness in general (Prolegomena 29). Kant attempts to remove Humes doubt by discussing the way the mind organizes its intuitions. Subjectively the mind senses things in a certain order, which it then rearranges according to a priori concepts of the understanding, so that the order in objective time obeys necessary laws.
For this rearrangement to be valid in objective time, the mind develops a set of universal laws which relate the connection of intuitions in objective time. These laws determine what experience is possible: If the empirical determination in relative time is to be objectively valid (i. e. , experience), these universal laws contain the necessity of the determination of existence in time generally. Experience gives to empirical judgments universal validity, and for that purpose requires a pure and a priori unity of the understanding (Prolegomena 26).
Causality is an example of such an a priori unity. Kant goes on to state explicitly that the concept of cause is a concept firmly established a priori, or before all experience, and have their undoubted objective value, though only with regard to experience (Prolegomena 27). Kant states that Hume justly maintains that we cannot comprehend by reason the possibility of causality, that is, of the reference of the existence of one thing to the existence of another which is necessitated by the former . . . But I am very far from holding these concepts to be derived merely from experience, and the necessity represented in them to be imaginary and a mere illusion produced in us by long habit. On the contrary, I have amply shown that they and the principles derived from them are firmly established a priori before all experience and have their undoubted objective value, though only with regard to experience (Prolegomena 27).
The explanation thus far cannot stand alone because although it explains how the a priori basis of cause is possible, it lacks a description of exactly why and how the specific a priori synthesis is made. Kant admits that although he cannot comprehend the concept of a necessary connection in either noumenal things or even appearances, we have yet a concept of such a connection of representations in our understanding and in judgments generally (Prolegomena 28).
He claims that a valid cognition of an object in representations depends on the minds a priori synthesis of concepts of the understanding. Kant gives three examples of such concepts, relationships, and connections of the understanding: the relationship of subject to predicate, the relationship of ground to subsequent, and parts of a whole. He says that necessarily the only way the human mind can organize its understanding of the phenomenal world is by making judgments which include one of these concepts. He says . . . the question is not how things in themselves but how the empirical cognition of things is determined, as regards the above moments of judgments in general, that is, how things, as objects of experience, can and must be subsumed under these concepts of the understanding. And then it is clear that I completely comprehend, not only the possibility, but also the necessity, of subsuming all appearances under these concepts, that is, of using them as principles of the possibility of experience (Prolegomena 28).
Thus, cause is a principle of the possibility of experience, an a priori synthesis necessary for the organization of intuitions into a coherent understanding of the phenomenal world. Kant, in order to clarify the theoretical explanation of a priori logic, explains in 29 exactly how the mind might proceed in a specific example. The mind sees the sun shining, an object illuminated by the sun, and senses that the object is growing warm. Constant conjunction and temporal succession, two of the three conditions for the validity of the concept of cause, have been satisfied.
The mind makes a hypothetical judgment, merely a subjective connection of perceptions (Prolegomena 29), that the light of the sun causes the heat in the object. The empirical rule of relation that one event always followed another in the past is made, with the hypothetical judgment, into a universal law. The mind makes this leap in the initial process of organizing the intuitions. The judgment of the intuitions, in order to qualify as experience, is taken with universal validity and places upon the intuitions from an objective standpoint the necessary connection that illuminates the concept of cause.
Thus for Kant, in order to develop experience, we must first make a priori synthesis which yield concepts of the understanding, including causality. He maintains that although one cannot form rationally the concept of one thing causing another, I therefore easily comprehend the concept of cause as a concept necessarily belonging to the mere form of experience, and its possibility as a synthetic unification of perceptions in a consciousness in general (Prolegomena 29).
In the Prolegomena, Kant solves both the problem of cause and the problem of induction. He does not try to find a corresponding sense impression for cause in experience, as Hume does. He does not need experience to find Humes foundation for the human minds necessity for the concept of cause. A priori concepts of the understanding, such as causation, are both possible and necessary for experience; they are not derived from experience, but experience is derived from them (Prolegomena 30).