The sensations aroused by light in the nervous mechanism of vision enable us to form conceptions as to the existence, form and position of external objects. Our ideas of things cannot be anything but symbols, natural signs for things which we learn how to use in order to regulate our movements and actions. Having learned correctly how to read those symbols, we are enabled by their help to adjust our actions so as to bring about the desired result; that is, so that the expected new sensations will arise.
There is no sense in asking whether vermilion as we see it, is really red, or whether this is simply an illusion of the senses. The sensation of red is the normal reaction of normally formed eyes to light reflected from vermilion. A person who is red-blind will see vermilion as black or as a dark grey-yellow. This too is the correct reaction for an eye formed in the special way his is. All he has to know is that his eye is simply formed differently from that of other persons. In itself the one sensation is not more correct and not more false than the other, although those who call this substance red are in the large majority.
In general, the red colour of vermilion exists merely in so far as there are eyes which are constructed like those of most people. Persons who are red-blind have just as much right to consider that a characteristic property of vermilion is that of being black. As a matter of fact, we should not speak of the light reflected from vermilion as being red, because it is not red except for certain types of eyes.
Herman von Helmholtz – Physiological Optics