How can we doubt, we say, that it exists?
The same external observation proves to us at once that there exists an object distinct from our nerves, and that our nerves separate us from it.

I insist on this point, for the reason that some authors, after having unreservedly admitted that our knowledge is confined to sensations, have subsequently been hard put to it to demonstrate the reality of the excitant distinct from the sensations.[6] Of this we need no demonstration, and the testimony of our senses suffices.

We have seen the excitant, and it is like a friend who should pass before us in disguise so well costumed and made up that we can attribute to his real self nothing of what we see of him, but yet we know that it is he.
And, in fact, let us remember what it is that we have argued upon--viz.

on an observation.

I look at my hand, and I see an object approaching it which gives me a sensation of feeling.