V. The existence of vacuum is a metaphysical question which does not belong to the regions of experience, and is not affected by the system of the sciences of observation.
103. By making the idea of space consist in abstract or generalized extension we reconcile all that is necessary, absolute, and infinite in it with its objective reality. This reality is the extension of bodies, while necessity and infinity are not found in the bodies themselves, but in the abstract idea. Objects themselves are confined to the sphere of reality, and are, therefore, limited and contingent. The objectiveness of the abstract idea includes both the existent and the possible, and has, therefore, no limits, and is not subject to any contingency.
[CHAPTER XVI.]
OBSERVATIONS ON KANT'S OPINION.
104. We have already shown that extension considered in us, is something more than a mere sensation, that it is a true idea, the basis of some sensations, and at the same time a pure idea. As far as it relates to sensations, it is the foundation of our sensitive faculties; and in so far as it is an idea, it is the root of geometry. This is an important distinction, and we shall find it useful to enable us rightly to appreciate the value of Kant's opinion of space.
105. All our sensations are, either more or less, connected with extension; although if we consider sensation a priori by itself, and independently of all habit, it would seem as though only the sensations of sight and touch were necessarily connected with an extended object. It does not seem to me that the loss of these two senses would necessarily involve the privation of the impressions of hearing or smelling, or, perhaps, even of taste; for although it is true that the sensations of touch, such as hardness or softness, etc., are always united with the sensations of the palate; it is equally certain that those sensations are wholly distinct from the sensation of taste, and we have no reason for asserting that they cannot be separated from it.
106. Extension, considered in us or in its intuition, may be regarded as a necessary condition of our sensitive faculties. Kant saw this, but he exaggerated it when he denied the objective reality of space, asserting that space is only a subjective condition a priori without which we cannot receive impressions, the form of phenomena, that is, of appearances, but nothing in reality. I have already said that space, as distinguished from bodies, is nothing, but the object of the idea of space is the extension of bodies; or, rather, this extension is the foundation from which we deduce the general idea of space, and is contained in this idea.
107. To say, as Kant does, that space is the form under which the phenomena are presented to us, and that it is a necessary subjective condition of their perception, is equivalent to saying that the phenomena which are presented as extended, require that the mind should be capable of perceiving extension. This is very true, but it throws no light on the nature of the idea of space, either in itself or in its object. "Space," says Kant, "is no empirical conception which is derived from external experience. For in order that certain sensations may be referred to something out of me, that is, to something in another part of space than that in which I am, and in order that I may conceive them as outside of and near one another, and, consequently, not only as separated, but also as occupying separate places, the conception of space must be placed as the foundation. Therefore, the conception of space cannot be obtained by experience from the relations of the external phenomenon, but this external experience itself is possible only by this conception."[48]
There is a great confusion of ideas here. What are the conditions which are necessary to the phenomenon of the sensation of the extended? We are not here treating of the appreciation of dimensions, but merely of extension as represented or conceived. I do not see how this phenomenon requires any thing a prior, except the sensitive faculty which, in fact, exists a prior, that is to say, is a primitive fact of our soul in its relations to the organization of the body which is united to it, and of the other bodies which surround it. Under certain conditions of our organization, and of the bodies which affect it, the soul receives the impressions of sight or touch, and with them the impression of extension. This extension is not presented to the mind in the abstract, or as separated from the other sensation which accompany it, but as united with them. The mind does not reflect, then, upon the position of the objects, but it has an intuition of the arrangement of the parts. So long as the fact is confined to mere sensation, it is common to the learned and the unlearned, to the old and the young, and even to all animals. This requires nothing a prior except the sensitive faculty, which simply means that a being, in order to perceive, must have the faculty of perceiving, and should hardly deserve to be announced as a discovery of philosophy.