I see a dog pass in the street.

I call this dog an external object; but, as this dog is formed, for me who am looking at it, of my sensations, and as these sensations are states of my nervous centres, it happens that the term external object has two meanings.

Sometimes it designates our sensations; at another, the exciting cause of our sensations.

To avoid all confusion we will call this exciting cause, which is unknown to us, the _X_ of matter.
It is, however, not entirely unknown, for we at least know two facts with regard to it.

We know, first, that this _X_ exists, and in the second place, that its image must not be sought in the sensations it excites in us.