The Treatise has been already spoken of as embracing two great objects, metaphysics and ethics; or three, if politics be considered as distinct from ethics. The great leading principle of the metaphysical department, and a principle which is never lost sight of in any part of the book, is, that the materials on which intellect works are the _impressions_ which represent immediate sensation, whether externally as by the senses, or internally as by the passions, and _ideas_ which are the faint reflections of these impressions. Thus to speak colloquially, when I see a picture, or when I am angry with some one, there is an _impression_; but when I think about this picture in its absence, or call to recollection my subsided anger, what exists in either case is an _idea_. Hume looked from words to that which they signified, and he found that where they signified any thing, it must be found among the things that either are or have been impressions. The whole varied and complex system of intellectual machinery he found occupied in the representation, the combination, or the arrangement of these raw materials of intellectual matter. If I say I see an object, I give expression to the fact, that a certain impression is made on the retina of my eye. If I convey to the person I am speaking to an accurate notion of what I mean, I awaken in his mind ideas left there by previous impressions, brought thither by his sense of sight.[70:1] Thus, in the particular case of the external senses, when they are considered as in direct communication between the mind and any object, there are impressions: when the senses are not said to be in communication with the object, the operations of the mind in connexion with it, are from vestiges which the impressions have left on the mind; and these vestiges are called ideas, and are always more faint than the original impressions themselves. And a material circumstance to be kept in view at the very threshold of the system is, that there is no specific and distinct line drawn between impressions and ideas. Their difference is in degree merely--the former are stronger, the latter weaker. There is no difference in kind; and there is sometimes doubt whether that which is supposed to be an impression may not be a vivid idea, and that which is supposed to be an idea a faint impression.