the mechanical effects of the matter, position, form, and motion of imagined atoms, and thus seeks to reduce all the forces of nature to action and reaction as its thing-in-itself. According to this teaching, light is the mechanical vibration or undulation of an imaginary ether, postulated for this end. This ether, if it reaches the eye, beats rapidly upon the retina, and gives us the knowledge of colour. Thus, for example, four hundred and eighty-three billion beats in a second give red, and seven hundred and twenty-seven billion beats in a second give violet. Upon this theory, persons who are colour-blind must be those who are unable to count the beats, must they not? Such crass, mechanical, clumsy, and certainly knotty theories, which remind one of Democritus, are quite worthy of those who, fifty years after the appearance of Goethe’s doctrine of colour, still believe in Newton’s homogeneous light, and are not ashamed to say so. They will find that what is overlooked in the child (Democritus) will not be forgiven to the man. They might indeed, some day, come to an ignominious end; but then every one would slink away and pretend that he never had anything to do with them. We shall soon have to speak again of this false reduction of the forces of nature to each other; so much for the present. Supposing this theory were possible, all would certainly be explained and established and finally reduced to an arithmetical problem, which would then be the holiest thing in the temple of wisdom, to which the principle of sufficient reason would at last have happily conducted us. But all content of the phenomenon would have disappeared, and the mere form would remain. The “what appears” would be referred to the “how it appears,” and this “how” would be what is _a priori_ knowable, therefore entirely dependent on the subject, therefore only for the subject, therefore, lastly, mere phantom, idea and form of idea, through and through: no thing-in-itself could be demanded. Supposing, then, that this were possible, the whole world would be derived from the subject, and in fact, that would be accomplished which Fichte wanted to _seem_ to accomplish by his empty bombast. But it is not possible: phantasies, sophisms, castles in the air, have been constructed in this way, but science never. The many and multifarious phenomena in nature have been successfully referred to particular original forces, and as often as this has been done, a real advance has been made. Several forces and qualities, which were at first regarded as different, have been derived from each other, and thus their number has been curtailed. (For example, magnetism from electricity.) Etiology will have reached its goal when it has recognised and exhibited as such all the original forces of nature, and established their mode of operation, _i.e._, the law according to which, under the guidance of causality, their phenomena appear in time