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Again, we may compare the truths of space with other truths derived from experience, which seem to have a necessity to us in proportion to the frequency of their recurrence or the truth of the consequences which may be inferred from them. We are thus led to remark that the necessity in our ideas of space on which much stress has been laid, differs in a slight degree only from the necessity which appears to belong to other of our ideas, e.g. weight, motion, and the like. And there is another way in which this necessity may be explained. We have been taught it, and the truth which we were taught or which we inherited has never been contradicted in all our experience and is therefore confirmed by it. Who can resist an idea which is presented to him in a general form in every moment of his life and of which he finds no instance to the contrary? The greater part of what is sometimes regarded as the a priori intuition of space is really the conception of the various geometrical figures of which the properties have been revealed by mathematical analysis. And the certainty of these properties is immeasurably increased to us by our finding that they hold good not only in every instance, but in all the consequences which are supposed to flow from them.

Neither must we forget that our idea of space, like our other ideas, has a history. The Homeric poems contain no word for it; even the later Greek philosophy has not the Kantian notion of space, but only the definite 'place' or 'the infinite.' To Plato, in the Timaeus, it is known only as the 'nurse of generation.' When therefore we speak of the necessity of our ideas of space we must remember that this is a necessity which has grown up with the growth of the human mind, and has been made by ourselves. We can free ourselves from the perplexities which are involved in it by ascending to a time in which they did not as yet exist. And when space or time are described as 'a priori forms or intuitions added to the matter given in sensation,' we should consider that such expressions belong really to the 'pre-historic study' of philosophy, i.e. to the eighteenth century, when men sought to explain the human mind without regard to history or language or the social nature of man.

In every act of sense there is a latent perception of space, of which we only become conscious when objects are withdrawn from it. There are various ways in which we may trace the connexion between them. We may think of space as unresisting matter, and of matter as divided into objects; or of objects again as formed by abstraction into a collective notion of matter, and of matter as rarefied into space. And motion may be conceived as the union of there and not there in space, and force as the materializing or solidification of motion. Space again is the individual and universal in one; or, in other words, a perception and also a conception. So easily do what are sometimes called our simple ideas pass into one another, and differences of kind resolve themselves into differences of degree.

Within or behind space there is another abstraction in many respects similar to it—time, the form of the inward, as space is the form of the outward. As we cannot think of outward objects of sense or of outward sensations without space, so neither can we think of a succession of sensations without time. It is the vacancy of thoughts or sensations, as space is the void of outward objects, and we can no more imagine the mind without the one than the world without the other. It is to arithmetic what space is to geometry; or, more strictly, arithmetic may be said to be equally applicable to both. It is defined in our minds, partly by the analogy of space and partly by the recollection of events which have happened to us, or the consciousness of feelings which we are experiencing. Like space, it is without limit, for whatever beginning or end of time we fix, there is a beginning and end before them, and so on without end. We speak of a past, present, and future, and again the analogy of space assists us in conceiving of them as coexistent. When the limit of time is removed there arises in our minds the idea of eternity, which at first, like time itself, is only negative, but gradually, when connected with the world and the divine nature, like the other negative infinity of space, becomes positive. Whether time is prior to the mind and to experience, or coeval with them, is (like the parallel question about space) unmeaning. Like space it has been realized gradually: in the Homeric poems, or even in the Hesiodic cosmogony, there is no more notion of time than of space. The conception of being is more general than either, and might therefore with greater plausibility be affirmed to be a condition or quality of the mind. The a priori intuitions of Kant would have been as unintelligible to Plato as his a priori synthetical propositions to Aristotle. The philosopher of Konigsberg supposed himself to be analyzing a necessary mode of thought: he was not aware that he was dealing with a mere abstraction. But now that we are able to trace the gradual developement of ideas through religion, through language, through abstractions, why should we interpose the fiction of time between ourselves and realities? Why should we single out one of these abstractions to be the a priori condition of all the others? It comes last and not first in the order of our thoughts, and is not the condition precedent of them, but the last generalization of them. Nor can any principle be imagined more suicidal to philosophy than to assume that all the truth which we are capable of attaining is seen only through an unreal medium. If all that exists in time is illusion, we may well ask with Plato, 'What becomes of the mind?'

Leaving the a priori conditions of sensation we may proceed to consider acts of sense. These admit of various degrees of duration or intensity; they admit also of a greater or less extension from one object, which is perceived directly, to many which are perceived indirectly or in a less degree, and to the various associations of the object which are latent in the mind. In general the greater the intension the less the extension of them. The simplest sensation implies some relation of objects to one another, some position in space, some relation to a previous or subsequent sensation. The acts of seeing and hearing may be almost unconscious and may pass away unnoted; they may also leave an impression behind them or power of recalling them. If, after seeing an object we shut our eyes, the object remains dimly seen in the same or about the same place, but with form and lineaments half filled up. This is the simplest act of memory. And as we cannot see one thing without at the same time seeing another, different objects hang together in recollection, and when we call for one the other quickly follows. To think of the place in which we have last seen a thing is often the best way of recalling it to the mind. Hence memory is dependent on association. The act of recollection may be compared to the sight of an object at a great distance which we have previously seen near and seek to bring near to us in thought. Memory is to sense as dreaming is to waking; and like dreaming has a wayward and uncertain power of recalling impressions from the past.