The outer world cannot be summarised in a few nervous systems suspended like spiders in empty space.

The existence of a nervous system implies that of a body in which it is lodged.

This body must have complicated organs; its limbs presuppose the soil on which the animal rests, its lungs the existence of oxygen vivifying its blood, its digestive tube, aliments which it digests and assimilates to its substance, and so on.

We may indeed admit that this outer world is not, in itself, exactly as we perceive it; but we are compelled to recognise that it exists by the same right as the nervous system, in order to put it in its proper place.
The second fact of observation is that the sensations we feel do not give us the true image of the material _X_ which produces them.

The modification made in our substance by this force _X_ does not necessarily resemble in its nature the nature of that force.