In any object submitted to the sight, what quality relating to the touch is necessary in order to produce the idea of extension? If we examine it closely, we shall find that there is none. Let us take a liquid; is its fluidity the necessary quality? No; for when congealed extension remains. Is it heat or cold? No; for without destroying its extension we may change its temperature as much as we please, no alteration is perceptible. Whatever quality relative to touch we may take, we shall find that it may be varied, modified, or entirely destroyed, without visibly affecting the extension. It often happens that we have a clear and definite idea of the extension of an object without knowing any thing of its qualities in relation to touch. I see an object at a distance, I distinguish its color and its form, but I know not of what material it is, whether it is of marble, or wood, or wax, nor whether this material is hard or soft, moist or dry, warm or cold. I do not even know if it is tangible, as in the case of figures formed by vapors which are imperceptible to the touch. 7. Without extension there can be neither sight, nor touch, nor any other sensation. As to taste, it is clear that it requires touch, and cannot exist without it. Our assertion is less clear with regard to sound and smell; for, although we cannot separate these sensations from the idea of extension as they always involve this idea in one way or another, we do not know how it would be with a man who was deprived of all the other senses, and retained only those of smelling and hearing. But without speculating on this hypothesis, it is enough to know: I. That nothing which is not extended can act upon our organs, unless by means wholly unknown to us, and which would give no idea of what we understand by body. II. That even supposing the sensations of smelling and hearing to be possible without the idea of extension, they would in that case be only simple phenomena of our being, and would not place us in communication with the external world, as we now perceive it; because, if we should not know that they proceeded from another cause, we could have no more consciousness of them than that which we have of the _me_; and if we should know it, this cause would be represented to us only as an agent influencing us, and not by any means as a being having any thing similar to what we understand by body. III. That in such a case we should have no idea of our own organization, nor of the universe; for it is clear that every thing being reduced to mere internal phenomena, and their relation to the agents producing them, and the idea of extension wanting, neither the universe nor our own body would be to us what they now are. What would the universe,--what would our body be without extension?