The distinction between the sensibility and the understanding[1] is to Kant fundamental both in itself and in relation to the conclusions which he reaches. An outline, therefore, of this distinction must precede any statement or examination of the details of his position. Unfortunately, in spite of its fundamental character, Kant never thinks of questioning or criticizing the distinction in the form in which he draws it, and the presence of certain confusions often renders it difficult to be sure of his meaning.

The distinction may be stated in his own words thus: "There are two stems of human knowledge, which perhaps spring from a common but to us unknown root, namely sensibility and understanding."[2] "Our knowledge springs from two fundamental sources of the mind; the first receives representations[3] (receptivity for impressions); the second is the power of knowing an object by means of these representations (spontaneity of conceptions). Through the first an object is given to us; through the second the object is thought in relation to the representation (which is a mere determination of the mind). Perception and conceptions constitute, therefore, the elements of all our knowledge, so that neither conceptions without a perception in some way corresponding to them, nor perception without conceptions can yield any knowledge.... Neither of these qualities has a preference over the other. Without sensibility no object would be given to us, and without understanding no object would be thought. Thoughts without content are empty, perceptions without conceptions are blind. Hence it is as necessary for the mind to make its conceptions sensuous (i. e. to add to them the object in perception) as to make its perceptions intelligible (i. e. to bring them under conceptions). Neither of these powers or faculties can exchange its function. The understanding cannot perceive, and the senses cannot think. Only by their union can knowledge arise."[4]

The distinction so stated appears straightforward and, on the whole,[5] sound. And it is fairly referred to by Kant as the distinction between the faculties of perceiving and conceiving or thinking, provided that the terms perceiving and conceiving or thinking be taken to indicate a distinction within perception in the ordinary sense of the word. His meaning can be stated thus: 'All knowledge requires the realization of two conditions; an individual must be presented to us in perception, and we as thinking beings must bring this individual under or recognize it as an instance of some universal. Thus, in order to judge 'This is a house' or 'That is red' we need the presence of the house or of the red colour in perception, and we must 'recognize' the house or the colour, i. e. apprehend the individual as a member of a certain kind. Suppose either condition unrealized. Then if we suppose a failure to conceive, i. e. to apprehend the individual as a member of some kind, we see that our perception—if it could be allowed to be anything at all—would be blind i. e. indeterminate, or a mere 'blur'. What we perceived would be for us as good as nothing. In fact, we could not even say that we were perceiving. Again, if we suppose that we had merely the conception of a house, and neither perceived nor had perceived an individual to which it applied, we see that the conception, being without application, would be neither knowledge nor an element in knowledge. Moreover, the content of a conception is derived from perception; it is only through its relation to perceived individuals that we become aware of a universal. To know the meaning of 'redness' we must have experienced individual red things; to know the meaning of 'house' we must at least have had experience of individual men and of their physical needs. Hence 'conceptions' without 'perceptions' are void or empty. The existence of conceptions presupposes experience of corresponding individuals, even though it also implies the activity of thinking in relation to these individuals.'[6]

Further, it is true to say that as perceiving we are passive; we do not do anything. This, as has been pointed out, is the element of truth contained in the statement that objects are given to us. On the other hand, it may be truly said that as conceiving, in the sense of bringing an individual under a universal, we are essentially active. This is presupposed by the notice or attention involved in perception ordinarily so called, i. e. perception in the full sense in which it includes conceiving as well as perceiving.[7] Kant, therefore, is justified in referring to the sensibility as a 'receptivity' and to the understanding as a 'spontaneity'.

The distinction, so stated, appears, as has been already said, intelligible and, in the main[8], valid. Kant, however, renders the elucidation of his meaning difficult by combining with this view of the distinction an incompatible and unwarranted theory of perception. He supposes,[9] without ever questioning the supposition, that perception is due to the operation of things outside the mind, which act upon our sensibility and thereby produce sensations. On this supposition, what we perceive is not, as the distinction just stated implies, the thing itself, but a sensation produced by it. Consequently a problem arises as to the meaning on this supposition of the statements 'by the sensibility objects are given to us' and 'by the understanding they are thought'. The former statement must mean that when a thing affects us there is a sensation. It cannot mean that by the sensibility we know that there exists a thing which causes the sensation, for this knowledge would imply the activity of thinking; nor can it mean that in virtue of the sensibility the thing itself is presented to us. The latter statement must mean that when sensation arises, the understanding judges that there is something causing it; and this assertion must really be a priori, because not dependent upon experience. Unfortunately the two statements so interpreted are wholly inconsistent with the account of the functions of the sensibility and the understanding which has just been quoted.

Further, this theory of perception has two forms. In its first form the theory is physical rather than metaphysical, and is based upon our possession of physical organs. It assumes that the reality to be apprehended is the world of space and time, and it asserts that by the action of bodies upon our physical organs our sensibility is affected, and that thereby sensations are originated in us. Thereupon a problem arises. For if the contribution of the sensibility to our knowledge of the physical world is limited to a succession of sensations, explanation must be given of the fact that we have succeeded with an experience confined to these sensations in acquiring knowledge of a world which does not consist of sensations.[10] Kant, in fact, in the Aesthetic has this problem continually before him, and tries to solve it. He holds that the mind, by means of its forms of perception and its conceptions of the understanding, superinduces upon sensations, as data, spatial and other relations, in such a way that it acquires knowledge of the spatial world.

An inherent difficulty, however, of this 'physical' theory of perception leads to a transformation of it. If, as the theory supposes, the cause of sensation is outside or beyond the mind, it cannot be known. Hence the initial assumption that this cause is the physical world has to be withdrawn, and the cause of sensation comes to be thought of as the thing in itself of which we can know nothing. This is undoubtedly the normal form of the theory in Kant's mind.

It may be objected that to attribute to Kant at any time the physical form of the theory is to accuse him of an impossibly crude confusion between things in themselves and the spatial world, and that he can never have thought that the cause of sensation, being as it is outside the mind, is spatial. But the answer is to be found in the fact that the problem just referred to as occupying Kant's attention in the Aesthetic is only a problem at all so long as the cause of sensation is thought of as a physical body. For the problem 'How do we, beginning with mere sensation, come to know a spatial and temporal world?' is only a problem so long as it is supposed that the cause of sensation is a spatial and temporal world or a part of it, and that this world is what we come to know. If the cause of sensation, as being beyond the mind, is held to be unknowable and so not known to be spatial or temporal, the problem has disappeared. Corroboration is given by certain passages[11] in the Critique which definitely mention 'the senses', a term which refers to bodily organs, and by others[12] to which meaning can be given only if they are taken to imply that the objects which affect our sensibility are not unknown things in themselves, but things known to be spatial. Even the use of the plural in the term 'things in themselves' implies a tendency to identify the unknowable reality beyond the mind with bodies in space. For the implication that different sensations are due to different things in themselves originates in the view that different sensations are due to the operation of different spatial bodies.

It is now necessary to consider how the distinction between the sensibility and the understanding contributes to articulate the problem 'How are a priori synthetic judgements possible?' As has been pointed out, Kant means by this question, 'How is it possible that the mind is able, in virtue of its own powers, to make universal and necessary judgements which anticipate its experience of objects?' To this question his general answer is that it is possible and only possible because, so far from ideas, as is generally supposed, having to conform to things, the things to which our ideas or judgements relate, viz. phenomena, must conform to the nature of the mind. Now, if the mind's knowing nature can be divided into the sensibility and the understanding, the problem becomes 'How is it possible for the mind to make such judgements in virtue of its sensibility and its understanding?' And the answer will be that it is possible because the things concerned, i. e. phenomena, must conform to the sensibility and the understanding, i. e. to the mind's perceiving and thinking nature. But both the problem and the answer, so stated, give no clue to the particular a priori judgements thus rendered possible nor to the nature of the sensibility and the understanding in virtue of which we make them. It has been seen, however, that the judgements in question fall into two classes, those of mathematics and those which form the presuppositions of physics. And it is Kant's aim to relate these classes to the sensibility and the understanding respectively. His view is that mathematical judgements, which, as such, deal with spatial and temporal relations, are essentially bound up with our perceptive nature, i. e. with our sensibility, and that the principles underlying physics are the expression of our thinking nature, i. e. of our understanding. Hence if the vindication of this relation between our knowing faculties and the judgements to which they are held to give rise is approached from the side of our faculties, it must be shown that our sensitive nature is such as to give rise to mathematical judgements, and that our understanding or thinking nature is such as to originate the principles underlying physics. Again, if the account of this relation is to be adequate, it must be shown to be exhaustive, i. e. it must be shown that the sensibility and the understanding give rise to no other judgements. Otherwise there may be other a priori judgements bound up with the sensibility and the understanding which the inquiry will have ignored. Kant, therefore, by his distinction between the sensibility and the understanding, sets himself another problem, which does not come into sight in the first formulation of the general question 'How are a priori synthetic judgements possible?' He has to determine what a priori judgements are related to the sensibility and to the understanding respectively. At the same time the distinction gives rise to a division within the main problem. His chief aim is to discover how it is that a priori judgements are universally applicable. But, as Kant conceives the issue, the problem requires different treatment according as the judgements in question are related to the sensibility or to the understanding. Hence arises the distinction between the Transcendental Aesthetic and the Transcendental Analytic, the former dealing with the a priori judgements of mathematics, which relate to the sensibility, and the latter dealing with the a priori principles of physics, which originate in the understanding. Again, within each of these two divisions we have to distinguish two problems, viz. 'What a priori judgements are essentially related to the faculty in question?' and 'How is it that they are applicable to objects?'

It is important, however, to notice that the distinction between the sensibility and the understanding, in the form in which it serves as a basis for distinguishing the Aesthetic and the Analytic, is not identical with or even compatible with the distinction, as Kant states it when he is considering the distinction in itself and is not thinking of any theory which is to be based upon it. In the latter case the sensibility and the understanding are represented as inseparable faculties involved in all knowledge.[13] Only from the union of both can knowledge arise. But, regarded as a basis for the distinction between the Aesthetic and the Analytic, they are implied to be the source of different kinds of knowledge, viz. mathematics and the principles of physics. It is no answer to this to urge that Kant afterwards points out that space as an object presupposes a synthesis which does not belong to sense. No doubt this admission implies that even the apprehension of spatial relations involves the activity of the understanding. But the implication is really inconsistent with the existence of the Aesthetic as a distinct part of the subject dealing with a special class of a priori judgements.