Cause and Effect.
The ideas of Cause and Effect belong to those first principles that reason draws from itself, and the category of causality is one of the most important. We never experience a sensation, of whatever kind, without attaching it, involuntarily and necessarily, to some external object which we know possesses the qualities corresponding to our sensations. Thus the impressions of heat or cold, sweet or bitter, blue or yellow, evoke immediately the picture of certain objects which are hot or cold, such as fire or ice; or which are sweet or bitter, such as sugar or absinth; or blue, as the sky; or yellow, as the lemon; and these external objects we consider as the causes, and our bodily sensations as the effects.