Take away the principle of causality, and the human mind is condemned never to go out of itself and its own modifications. All the sensations of hearing, of smell, of taste, of touch, of feeling even, cannot inform you what their cause is, nor whether they have a cause. But give to the human mind the principle of causality, admit that every sensation, as well as every phenomenon, every change, every event, has a cause, as evidently we are not the cause of certain sensations, and that especially these sensations must have a cause, and we are naturally led to recognize for those sensations causes different from ourselves, and that is the first notion of an exterior world. The universal and necessary principle of causality alone gives it and justifies it. Other principles of the same order increase and develop it. As soon as you know that there are external objects, I ask you whether you do not conceive them in a place that contains them. In order to deny it, it would be necessary to deny that every body is in a place, that is to say, to reject a truth of physics, which is at the same time a principle of metaphysics, as well as an axiom of common sense. But the place that contains a body is often itself a body, which is only more capacious than the first. This new body is in its turn in a place. Is this new place also a body? Then it is contained in another place more extended, and so on; so that it is impossible for you to conceive a body which is not in a place; and you arrive at the conception of a boundless and infinite place, that contains all limited places and all possible bodies: that boundless and infinite place is space. And I tell you in this nothing that is not very simple. Look. Do you deny that this water is in a vase? Do you deny that this vase is in this hall? Do you deny that this hall is in a larger place, which is in its turn in another larger still? I can thus carry you on to infinite space. If you deny a single one of these propositions, you deny all, the first as well as the last; and if you admit the first, you are forced to admit the last. It cannot be supposed that sensibility, which is not able to give us even the idea of body, alone elevates us to the idea of space. The intervention of a superior principle is, therefore, here necessary. As we believe that every body is contained in a place, so we believe that every event happens in time. Can you conceive an event happening, except in some point of duration? This duration is extended and successively increased to your mind's eye, and you end by conceiving it unlimited like space. Deny duration, and you deny all the sciences that measure it, you destroy all the natural beliefs upon which human life reposes. It is hardly necessary to add that sensibility alone no more explains the notion of time than that of space, both of which are nevertheless inherent in the knowledge of the external world.