This is what I shall now demonstrate as clearly as I can.
Take the case of an unlearned person.

To prove to him that he knows sensations alone and not the bodies which excite them, a very striking argument may be employed which requires no subtle reasoning and which appeals to his observation.

This is to inform him, supposing he is not aware of the fact, that, every time he has the perception of an exterior object, there is something interposed between the object and himself, and that that something is his nervous system.
If we were not acquainted with the existence of our nervous system, we should unhesitatingly admit that our perception of objects consisted in some sort of motion towards the places in which they were fixed.
Now, a number of experiments prove to us that objects are known to us as excitants of our nervous system which only act on this system by entering into communication, or coming into contact with, its terminal extremities.

They then produce, in the interior of this system, a peculiar modification which we are not yet able to define.

It is this modification which follows the course of the nerves and is carried to the central parts of the system.