I at first say that this object is an excitant.

It is pointed out to me that I am in error.

This object, which appears to me outside my nervous system, is composed, I am told, of sensations.

Be it so, I have the right to answer; but if all that I perceive is sensation, my nervous system itself is a sensation; if it is only that, it is no longer an intermediary between the excitant and myself, and it is the fact that we perceive things as they are.

For it to be possible to prove that I perceive, not the object, but that _tertium quid_ which is sensation, it has to be admitted that the nervous system is a reality external to sensation and that objects which assume, in relation to it, the role of excitants and of which we perceive the existence, are likewise realities external to sensation.
This is what is demonstrated by abstract reasoning, and this reasoning is further supported by a common-sense argument.