The arguments by which Kant seeks to show that time is not a determination of things in themselves but only a form of perception are, mutatis mutandis, identical with those used in his treatment of space.[1] They are, therefore, open to the same criticisms, and need no separate consideration.

Time, however, according to Kant, differs from space in one important respect. It is the form not of outer but of inner sense; in other words, while space is the form under which we perceive things, time is the form under which we perceive ourselves. It is upon this difference that attention must be concentrated. The existence of the difference at all is upon general grounds surprising. For since the arguments by which Kant establishes the character of time as a form of perception run pari passu with those used in the case of space, we should expect time, like space, to be a form under which we perceive things; and, as a matter of fact, it will be found that the only argument used to show that time is the form of inner, as opposed to outer, sense is not only independent of Kant's general theory of forms of sense, but is actually inconsistent with it.[2] Before, however, we attempt to decide Kant's right to distinguish between inner and outer sense, we must consider the facts which were before Kant's mind in making the distinction.

These facts and, to a large extent, the frame of mind in which Kant approached them, find expression in the passage in Locke's Essay, which explains the distinction between 'ideas of sensation' and 'ideas of reflection'.

"Whence has it [i. e. the mind] all the materials of reason and knowledge? To this I answer, in one word, from experience.... Our observation, employed either about external, sensible objects, or about the internal operations of our minds, perceived and reflected on, by ourselves, is that which supplies our understandings with all the materials of thinking. These two are the fountains of knowledge...."

"First, Our senses, conversant about particular sensible objects, do convey into the mind several distinct perceptions of things, according to those various ways, wherein those objects do affect them: and thus we come by those ideas we have of Yellow, White, Heat, Cold, Soft, Hard, Bitter, Sweet, and all those, which we call sensible qualities; which, when I say the senses convey into the mind, I mean, they, from external objects, convey into the mind what produces there those perceptions. This great source of most of the ideas we have, depending wholly upon our senses, and derived by them to the understanding, I call sensation."

"Secondly, The other fountain, from which experience furnisheth the understanding with ideas, is the perception of the operations of our own mind within us, as it is employed about the ideas it has got; which operations, when the soul comes to reflect on, and consider, do furnish the understanding with another set of ideas, which could not be had from things without; and such are Perception, Thinking, Doubting, Believing, Reasoning, Knowing, Willing, and all the different actings of our own minds; which we being conscious of, and observing in ourselves, do, from these, receive into our understandings as distinct ideas, as we do from bodies affecting our senses. This source of ideas every man has wholly in himself; and though it be not sense as having nothing to do with external objects, yet it is very like it, and might properly enough be called internal sense. But, as I call the other sensation, so I call this reflection; the ideas it affords being such only as the mind gets, by reflecting on its own operations within itself."[3]

Here Locke is thinking of the distinction between two attitudes of mind, which, however difficult it may be to state satisfactorily, must in some sense be recognized. The mind, undoubtedly, in virtue of its powers of perceiving and thinking—or whatever they may be—becomes through a temporal process aware of a spatial world in its varied detail. In the first instance, its attention is absorbed in the world of which it thus becomes aware; subsequently, however, it is in some way able to direct its attention away from this world to the activities in virtue of which it has become aware of this world, and in some sense to make itself its own object. From being conscious it becomes self-conscious. This process by which the mind turns its attention back upon itself is said to be a process of 'reflection'. While we should say that it is by perception that we become aware of things in the physical world, we should say that it is by reflection that we become aware of our activities of perceiving, thinking, willing, &c. Whatever difficulties the thought of self-consciousness may involve, and however inseparable, and perhaps even temporally inseparable, the attitudes of consciousness and self-consciousness may turn out to be, the distinction between these attitudes must be recognized. The object of the former is the world, and the object of the latter is in some sense the mind itself; and the attitudes may be described as that of our ordinary, scientific, or unreflecting consciousness and that of reflection.

The significance of Locke's account of this distinction lies for our purposes in its anticipation of Kant. He states the second attitude, as well as the first, in terms of sense. Just as in our apprehension of the world things external to, in the sense of existing independently of, the mind are said to act on our physical organs or 'senses', and thereby to produce 'perceptions' in the mind, so the mind is said to become conscious of its own operations by 'sense'. We should notice, however, that Locke hesitates to use the word 'sense' in the latter case, on the ground that it involves no operation of external things (presumably upon our physical organs), though he thinks that the difficulty is removed by calling the sense in question 'internal'.

Kant is thinking of the same facts, and also states them in terms of sense, though allowance must be made for the difference of standpoint, since for him 'sense', in the case of the external sense, refers not to the affection of our physical organs by physical bodies, but to the affection of the mind by things in themselves. Things in themselves act on our minds and produce in them appearances, or rather sensations, and outer sense is the mind's capacity for being so affected by outer things, i. e. things independent of the mind. This is, in essentials, Kant's statement of the attitude of consciousness, i. e. of our apprehension of the world which exists independently of the mind, and which, for him, is the world of things in themselves. He also follows Locke in giving a parallel account of the attitude of self-consciousness. He asks, 'How can the subject perceive itself?' Perception in man is essentially passive; the mind must be affected by that which it perceives. Consequently, if the mind is to perceive itself, it must be affected by its own activity; in other words, there must be an inner sense, i. e. a capacity in virtue of which the mind is affected by itself.[4] Hence Kant is compelled to extend his agnosticism to the knowledge of ourselves. Just as we do not know things, but only the appearances which they produce in us,[5] so we do not know ourselves, but only the appearances which we produce in ourselves; and since time is a mode of relation of these appearances, it is a determination not of ourselves, but only of the appearances due to ourselves.

The above may be said to represent the train of thought by which Kant arrived at his doctrine of time and the inner sense. It was reached by combining recognition of the fact that we come to be aware not only of the details of the physical world, but also of the successive process on our part by which we have attained this knowledge, with the view that our apprehension of this successive process is based on 'sense', just as is our apprehension of the world. But the question remains whether Kant is, on his own principles, entitled to speak of an inner sense at all. According to him, knowledge begins with the production in us of sensations, or, as we ought to say in the present context, appearances by the action of things in themselves. These sensations or appearances can reasonably be ascribed to external sense. They may be ascribed to sense, because they arise through our being affected by things in themselves. The sense may be called external, because the object affecting it is external to the mind, i. e. independent of it. In conformity with this account, internal sense must be the power of being affected by something internal to the mind, i. e. dependent upon the mind itself, and since being affected implies the activity of affecting, it will be the power of being affected by the mind's own activity.[6] The activity will presumably be that of arranging spatially the sensations or appearances due to things in themselves.[7] This activity must be said to produce an affection in us, the affection being an appearance due to ourselves. Lastly, the mind must be said to arrange these appearances temporally. Hence it will be said to follow that we know only the appearances due to ourselves and not ourselves, and that time is only a determination of these appearances.[8] The weakness of the position just stated lies on the surface. It provides no means of determining whether any affection produced in us is produced by ourselves rather than by the thing in itself; consequently we could never say that a given affection was an appearance due to ourselves, and therefore to inner sense. On the contrary, we should ascribe all affections to things in themselves, and should, therefore, be unable to recognize an inner sense at all. In order to recognize an inner sense we must know that certain affections are due to our activity, and, to do this, we must know what the activity consists in—for we can only be aware that we are active by being aware of an activity of ours of a particular kind—and, therefore, we must know ourselves. Unless, then, we know ourselves, we cannot call any affections internal.

If, however, the doctrine of an internal sense is obviously untenable from Kant's own point of view, why does he hold it? The answer is that, inconsistently with his general view, he continues to think of the facts as they really are, and that he is deceived by an ambiguity into thinking that the facts justify a distinction between internal and external sense.

He brings forward only one argument to show that time is the form of the internal sense. "Time is nothing else than the form of the internal sense, i. e. of the perception of ourselves and our inner state. For time cannot be any determination of external phenomena; it has to do neither with a shape nor a position; on the contrary, it determines the relation of representations in our internal state."[9] To follow this argument it is first necessary to realize a certain looseness and confusion in the expression of it. The term 'external', applied to phenomena, has a double meaning. It must mean (1) that of which the parts are external to one another, i. e. spatial; for the ground on which time is denied to be a determination of external phenomena is that it has nothing to do with a shape or a position. It must also mean (2) external to, in the sense of independent of, the mind; for it is contrasted with our internal state, and if 'internal', applied to 'our state', is not to be wholly otiose, it can only serve to emphasize the contrast between our state and something external to in the sense of independent of us. Again, 'phenomena,' in the phrase 'external phenomena', can only be an unfortunate expression for things independent of the mind, these things being here called phenomena owing to Kant's view that bodies in space are phenomena. Otherwise, 'phenomena' offers no contrast to 'our state' and to 'representations'. The passage, therefore, presupposes a distinction between states of ourselves and things in space, the former being internal to, or dependent upon, and the latter external to, or independent of, the mind.

It should now be easy to see that the argument involves a complete non sequitur. The conclusion which is justified is that time is a form not of things but of our own states. For the fact to which he appeals is that while things, as being spatial, are not related temporally, our states are temporally related; and if 'a form' be understood as a mode of relation, this fact can be expressed by the formula 'Time is a form not of things but of our own states', the corresponding formula in the case of space being 'Space is a form not of our states but of things'. But the conclusion which Kant desires to draw—and which he, in fact, actually draws—is the quite different conclusion that time is a form of perception of our states, the corresponding conclusion in the case of space being that space is a form of perception of things. For time is to be shown to be the form of inner sense, i. e. the form of the perception of what is internal to ourselves, i. e. of our own states.[10] The fact is that the same unconscious transition takes place in Kant's account of time which, as we saw,[11] takes place in his account of space. In the case of space, Kant passes from the assertion that space is a form of things, in the sense that all things are spatially related—an assertion which he expresses by saying that space is the form of phenomena—to the quite different assertion that space is a form of perception, in the sense of a way in which we perceive things as opposed to a way in which things are. Similarly, in the case of time, Kant passes from the assertion that time is the form of our internal states, in the sense that all our states are temporally related, to the assertion that time is a way in which we perceive our states as opposed to a way in which our states really are. Further, the two positions, which he thus fails to distinguish, are not only different, but incompatible. For if space is a form of things, and time is a form of our states, space and time cannot belong only to our mode of perceiving things and ourselves respectively, and not to the things and ourselves; for ex hypothesi things are spatially related, and our states are temporally related.

Kant's procedure, therefore, may be summed up by saying that he formulates a view which is true but at the same time inconsistent with his general position, the view, viz. that while things in space are not temporally related, the acts by which we come to apprehend them are so related; and further, that he is deceived by the verbally easy transition from a legitimate way of expressing this view, viz. that time is the form of our states, to the desired conclusion that time is the form of inner sense.

The untenable character of Kant's position with regard to time and the knowledge of ourselves can be seen in another way. It is not difficult to show that, in order to prove that we do not know things, but only the appearances which they produce, we must allow that we do know ourselves, and not appearances produced by ourselves, and, consequently, that time is real and not phenomenal. To show this, it is only necessary to consider the objection which Kant himself quotes against his view of time. The objection is important in itself, and Kant himself remarks that he has heard it so unanimously urged by intelligent men that he concludes that it must naturally present itself to every reader to whom his views are novel. According to Kant, it runs thus: "Changes are real (this is proved by the change of our own representations, even though all external phenomena, together with their changes, be denied). Now changes are only possible in time; therefore time is something real."[12] And he goes on to explain why this objection is so unanimously brought, even by those who can bring no intelligible argument against the ideality of space. "The reason is that men have no hope of proving apodeictically the absolute reality of space, because they are confronted by idealism, according to which the reality of external objects is incapable of strict proof, whereas the reality of the object of our internal senses (of myself and my state) is immediately clear through consciousness. External objects might be mere illusion, but the object of our internal senses is to their mind undeniably something real."[13]

Here, though Kant does not see it, he is faced with a difficulty from which there is no escape. On the one hand, according to him, we do not know things in themselves, i. e. things independent of the mind. In particular, we cannot know that they are spatial; and the objection quoted concedes this. On the other hand, we do know phenomena or the appearances produced by things in themselves. Phenomena or appearances, however, as he always insists, are essentially states or determinations of the mind. To the question, therefore, 'Why are we justified in saying that we do know phenomena, whereas we do not know the things which produce them?' Kant could only answer that it is because phenomena are dependent upon the mind, as being its own states.[14] As the objector is made to say, 'the reality of the object of our internal senses (of myself and my state) is immediately clear through consciousness.' If we do not know things in themselves, because they are independent of the mind, we only know phenomena because they are dependent upon the mind. Hence Kant is only justified in denying that we know things in themselves if he concedes that we really know our own states, and not merely appearances which they produce.

Again, Kant must allow—as indeed he normally does—that these states of ours are related by way of succession. Hence, since these states are really our states and not appearances produced by our states, these being themselves unknown, time, as a relation of these states, must itself be real, and not a way in which we apprehend what is real. It must, so to say, be really in what we apprehend about ourselves, and not put into it by us as perceiving ourselves.

The objection, then, comes to this. Kant must at least concede that we undergo a succession of changing states, even if he holds that things, being independent of the mind, cannot be shown to undergo such a succession; consequently, he ought to allow that time is not a way in which we apprehend ourselves, but a real feature of our real states. Kant's answer[15] does not meet the point, and, in any case, proceeds on the untenable assumption that it is possible for the characteristic of a thing to belong to it as perceived, though not in itself.[16]

[1] Cf. B. 46-9, §§ 4, 5 and 6 (a), M. 28-30, §§ 5, 6 and 7 (a) with B. 38-42, § 2 (1-4), and § (3) to (a) inclusive, M. 23-6, §§ 2, 3, and 4 (a). The only qualification needed is that, since the parts of time cannot, like those of space, be said to exist simultaneously, B. § 4 (5), M. § 5, 5 is compelled to appeal to a different consideration from that adduced in the parallel passage on space (B. § 2 (4), M. § 2, 4). Since, however, B. § 4 (5), M. § 5, 5 introduces no new matter, but only appeals to the consideration already urged (B. § 4, 4, M. § 5, 4), this difference can be neglected. B. § 5, M. § 6 adds a remark about change which does not affect the main argument.