In the light of these considerations, some of the views which natural science has developed during the nineteenth century appear for the first time in the right light. The supporters of these views tell us that we hear, see, and touch the objects of the physical world through our senses. The eye, for instance, transmits to us a phenomenon of light, a colour. Thus we say that a body emits red light, when with the help of the eye we experience the sensation “red.” But the eye can give us this same sensation in other cases also. If the eyeball is struck or pressed upon, or if an electric spark is allowed to pass through the head, the eye has a sensation of light. It is thus evident that even in the cases in which we have the sensation of a body emitting red light, something may really be happening in that body which has no sort of resemblance to the colour we sensate. Whatever may be actually happening “outside of us” in space, so long as what happens is capable of making an impression on the eye, there arises in us the sensation of light. Thus what we experience _arises in us_, because we possess organs constituted in a particular manner. What happens outside in space, remains outside of us; we know only the effects which the external happenings call up in us. Hermann Helmholtz (1821-1893) has given a clearly outlined expression to this thought: “Our sensations are simply effects which are produced in our organs by external causes, and the manner in which such an effect will show itself depends, naturally enough, altogether upon the kind of apparatus upon which the action takes place. In so far as the quality of our sensation gives us information as to the peculiar nature of the external action which produces the sensation, so far can the sensation be regarded as a sign or symbol of this external action, but not as an image or reproduction of it. For we expect in a picture some kind of resemblance to the object it represents; thus in a statue, resemblance of form; in a drawing, resemblance in the perspective projection of the field of view; in a painting, resemblance of colour in addition. A symbol, however, is not required to have any sort of resemblance to that which it symbolises. The necessary connection between the object and the symbol is limited to this: that the same object coming into action under the same conditions shall call up the same symbol, and that therefore different symbols shall always correspond to different objects. When berries of a certain kind in ripening produce together red colouration and sugar, then red colour and a sweet taste will always find themselves together in our sensation of berries of this form.”[14] [14] _Cp._ Helmholtz, _Die Thatsachen der Wahrnehmung_, p. 12 _et seq._ I have characterised this kind of conception in detail in my _Philosophie der Freiheit_, Berlin, 1894, and in my _Welt- und Lebensanschauungen im Neunzehnten Jahrhundert_, vol. ii., p. i., etc.