to certain phenomena a more general scope than is supported by the senses, for instance when it hastily attributes to what is in fact only an optical existence, a supposed plastic existence also. The judgment of error is a prejudice. Truth and error, understanding and misunderstanding, knowing and not knowing, have their common habitation in the faculty of thought which is the organ of science. Thought at large is the general expression of experienced facts perceived by the senses, and it includes errors as well. Error is distinguished from truth in that the former assigns to any definite fact of which it is a manifestation, a wider and more general existence than is supported by sense perceptions and experience. Unwarranted assumption is the nature of error. A glass bead does not become a counterfeit, until it pretends to be a genuine pearl. Schleiden says of the eye: "When the excited blood expands the veins and presses on the nerves, we feel it in the fingers as pain, we see it in the eyes as forked lightning. And thus we obtain the irrefutable proof that our conceptions are free creations of the mind, that we do not perceive the external world as it really is, but that its reflex actions on us simply give rise to a peculiar brain activity, on our part. The products of this activity are frequently connected with certain processes of the external world, but frequently they are not. We close our eyes and we see a circle of light, but there is in reality no shining body. It is easy to see that this may be a great and dangerous source of errors of all kinds. From the teasing forms of a misty moonlight night to the threatening and insanity-producing visions of the believer in ghosts we meet a series of illusions which are not derived from any direct processes of external nature, but belong to the field of the free activity of the mind which is subject to error. It requires great judgment and wide education, before the mind learns to break away from all its own errors and to control them. Reading in general seems so easy, and yet it is a difficult art. It is only by degrees that the mind learns to understand which of the messages of the nerves may be trusted and used as a basis for conceptions. The light, if we consider it entirely by itself, is not clear, not yellow, nor blue nor red. The light is a movement of a very fine and everywhere diffused substance, the ether."