they are not; not a capacity of knowledge, but of illusion.
But it is quite another matter when we look not at one colour alone but take several colours together and distinguish them from one another. Each of them is called forth by distinct ether waves of different lengths. To the distinctions in the colours there correspond differences in the length of the ether waves. These distinctions do not exist in my organ of vision, but have their ground in the external world. My organs of vision only have the functions of making me conscious of this difference in a certain form, that of colour. As a means to a recognition of this distinction it is a power of real knowledge and not of illusion. These distinctions are no mere appearances. The fact that I see green, red, and white has its ground in my organ of sight. But that the green should differ from the red, testifies to something that lies outside of me, to a real difference between the things.
Moreover, the peculiarity of my organ has the effect that by its means I can only recognise the motions of the ether. No other communication from the outer world can reach me through that medium.
Just as with the power of vision, in particular, so is it with the organs of knowledge in general. They can only convey to me space and time conceptions, that is, they can only show me those relations of the things which can call forth time and space conceptions in my head. To impressions of another kind, if there are any, they cannot react, and my faculty of knowledge renders it possible for me to obtain any impressions in a particular way. So far the categories of space and time are founded in the construction of my faculty of knowledge.
But the relations and distinctions of the things themselves, which are shown to me by means of the individul space and time concepts, so that the different things appear to me as big and small, near and far, sooner or later, are real relations and distinctions of the external world, which are not conditioned through the nature of my faculty of knowledge.
