Although when we conceive extension in general, in the abstract, considering it as a mere continuity, we absolutely abstract it from all form; when we have need of an applied extension as the recipient of sensations, it is impossible to find it without a determinate form and figure. We do not see a color simply, but we see it in a circular, triangular, or other extension. These forms are confounded with extension itself as its applications; and do not serve as a link for other sensations. Sometimes, it is true, the same figure receives different colors, different positions, different degrees of heat or cold, etc., but the contrary also sometimes occurs, and with the same color, and the same degree of heat or cold, with the same continuance of the other sensations, the object changes its form; just as a red circle may become a green circle, a red object may become circular, and afterwards triangular. In the first case, the circular figure is the link connecting the sensations of the colors; in the second, the color is the link connecting the figures. 12. Having deprived extension of the honor of substance, as well as all other sensations, in so far as objective; we may observe that all these variations in the objects are successive, and the sensations are connected with each other. Thus the same circle may take different colors; and the same color different figures; the colors may be again changed, and the first reproduced, the figure remaining the same; or the first figure may be reproduced, the colors remaining the same. We conclude that under this variety there is something constant, that under this multiplicity there is something which is one; that under this succession of being and not-being there is something permanent; and this which is constant, one, and permanent, the recipient of these changes, the point outside of us which connects them, and enables us to conceive them connected,--this is what we call substance. CHAPTER III. DEFINITION OF CORPOREAL SUBSTANCE. 13. What is the permanent subject of transformations in the sensible order? Is it a pure illusion? Is it a reality? What reality can it be? Does it not seem rather an abstraction? A thing which is no color, but lends itself all colors; which is none of the qualities which we experience, but the subject and cause of them all; which is no form, but accommodates itself to all forms; which is not pure extension, because this is an abstraction, and it is something which serves as the ground of other things; a corporeal object which, in itself, can affect none of the senses; what is it? Is it what the Aristotelians call an occult quality, a mysterious, and fantastic being, a mere illusion? Let us examine it by the light of experience.