It must be remembered that the synthesis of apprehension, as the "mind" of "sense," is itself a _double_, containing the pure conjunctive unity of "unconscious understanding" as an active factor, and susceptibility to impact as a passive factor. In the conjoined relation of these two factors every material phenomenon gets to exist; so there must be _some_ relation of space to _every_ external object, and to _all_ external objects--which is to say at once that space is _infinite_, both in extent and divisibility, so far as it can apply to objects _at all_.
And here, too, is the reason that the contained character, the constituent quality, of space--meaning what Kant termed the "form of the intuition"--is essentially plural. This constituent quality of space is a _re_-presentation of mind, as at once active and passive, receptive and reflexive--as fundamental _a priori_ self-separateness. But _space itself_, as a _whole_, is the _synthesis_ of this self-separateness. It is self-unity of self-separateness, _materialized_. Space, made of spaces, is a thing identical in form and contents. Kant said:
"Space _re_-presented as an _object_ (as geometry really requires it to be) contains more than the mere _form of the intuition_; namely, a combination of the manifold given according to the form of sensibility into a representation that can be intuited; so the _form of the intuition_ gives us merely the manifold, but the _formal intuition_ gives unity of re-presentation. In the 'Æsthetic,' [the first division of _The Critique of Pure Reason_], I regarded this unity as belonging entirely to sensibility, for the purpose of indicating that it antecedes all conceptions, although it presupposes a synthesis which does not belong to sense, through which, however, all our conceptions of space and time are possible.... By means of this unity alone (the understanding determining the sensibility) space and time are given as intuitions."
It is easy enough to follow out the genesis of time, in the same way as the genesis of space. The constituent quality of space and time is the same in both, and is subject in both to the same act of synthesis, in order that the essential plurality of "the form of intuition" may be created into the unity of "the formal intuition" itself--the single thing, space or time. But time is the "form" of "_in_-ternal sense," as Kant put it, while space is the "form" of "_ex_-ternal sense"--sense being to Kant not its physical organs (which are matter), but mental _susceptibility_ as distinguished from mental _synthesis_. Every phenomenon in space is made of active subjective-synthesis, passive subjective-susceptibility, and noumenal impact. Space and time themselves are made of the synthesis and the susceptibility alone. But pure synthesis, which means just pure identity of awareness, can _have_ no "susceptibility," cannot be _occupied_, without _change of state_; and any change of state in a pure general awareness forms succession of states, or, as Kant said, "_generates time_." But conjunction, again, of synthesis and susceptibility must be the relating of separates, with reference to the objective as well as the subjective factor. As objective effect this relation is pure co-existence of separates in time, through outness from each other--space. All objects, impressions, "effects of sense," must take the order of time; but "objects of internal sense" (feelings, or emotions), having no direct filling from noumena, are not objects in space. Thus, while space is pure synthesis of apprehension _ex_-ternally objected, time is the same pure synthesis of apprehension _in_-ternally objected.