Click on the phrases to see them in context. The original texts by Immanuel Kant and David Hume are available from the Gutenberg Projet.

On this successive synthesis of the productive imagination, in the generation of figures, is founded the mathematics of extension, or geometry, with its axioms, which express the conditions of sensuous intuition a priori, under which alone the schema of a pure conception of external intuition can exist; for example, "be tween two points only one straight line is possible," "two straight lines cannot enclose a space," etc.

The foregoing deduction is an exposition of the pure conceptions of the understanding (and with them of all theoretical a priori cognition), as principles of the possibility of experience, but of experience as the determination of all phenomena in space and time in general--of experience, finally, from the principle of the original synthetical unity of apperception, as the form of the understanding in relation to time and space as original forms of sensibility.

Space is merely the form of external intuition, but not a real object which can itself be externally intuited; it is not a correlate of phenomena, it is the form of phenomena itself.

At the same time, how "I who think" is distinct from the "I" which intuites itself (other modes of intuition being cogitable as at least possible), and yet one and the same with this latter as the same subject; how, therefore, I am able to say; "I, as an intelligence and thinking subject, cognize myself as an object thought, so far as I am, moreover, given to myself in intuition--only, like other phenomena, not as I am in myself, and as considered by the understanding, but merely as I appear"--is a question that has in it neither more nor less difficulty than the question--"How can I be an object to myself?" or this--"How I can be an object of my own intuition and internal perceptions?" But that such must be the fact, if we admit that space is merely a pure form of the phenomena of external sense, can be clearly proved by the consideration that we cannot represent time, which is not an object of external intuition, in any other way than under the image of a line, which we draw in thought, a mode of representation without which we could not cognize the unity of its dimension, and also that we are necessitated to take our determination of periods of time, or of points of time, for all our internal perceptions from the changes which we perceive in outward things. We must now consider three phaenomena, which seem to be, in a manner, the reverse of these: Why a very great distance encreases our esteem and admiration for an object; Why such a distance in time encreases it more than that in space: And a distance in past time more than that in future. It would be unjust to accuse us of holding the long-decried theory of empirical idealism, which, while admitting the reality of space, denies, or at least doubts, the existence of bodies extended in it, and thus leaves us without a sufficient criterion of reality and illusion.
This is what one had to say:
Whatever perceptions you may attain to, you are still surrounded by conditions--in space, or in time--and you cannot discover anything unconditioned; nor can you decide whether this unconditioned is to be placed in an absolute beginning of the synthesis, or in an absolute totality of the series without beginning. We cannot cogitate a geometrical line without drawing it in thought, nor a circle without describing it, nor represent the three dimensions of space without drawing three lines from the same point perpendicular to one another.