SENSATION IN ITSELF.
1. Sensation considered in itself is simply an internal affection; but it is almost always accompanied by a judgment. This judgment may be more or less explicit and more or less noticed by the subject of the sensation.
Suppose I see two architectural ornaments at a proper distance, and I discover no difference between them. In this sensation there are two things to be considered.
I. The internal affection which we call seeing. On this point all doubt is impossible. Whether I am asleep or awake, raving or in my sober senses, whether the ornaments are alike or unlike, whether, in fine, they exist or not, there still exists in my soul the representation which I call seeing the ornaments.
II. I also at the same time form a judgment that, besides the internal affection which I experience, the ornaments exist, that they are in relief, and that they are before my eyes. In this judgment I may be deceived; for I may be asleep or in a delirium; it may be that the ornaments are behind me, and that I only see their reflection in a mirror; it may be that what I see is only a paper placed in such a manner behind a glass, as to make upon the retina of my eyes the same impression as the ornaments would if they were really present; or, finally, it may all be the work of a skilful painter who has given this illusory appearance to his canvas.
From this we see that, admitting the existence of the internal fact of sensation, it is possible:
I. That there is no external object.
II. That the objects exists, but not in the position supposed.
III. That the object is not the architectural ornaments.
IV. That both are plane surfaces; or, that one is in relief, and the other a plane.