The things that concern man cannot be found through the study of animals. I have often stressed this difference in public lectures, and should like to emphasise it still more here. People are in the habit of thinking: an eye is an eye, an organ is an organ, lungs are lungs, a liver is a liver, and so forth. But that is not so, the eye in man is the organ which also exists in the animal world as eye, but with a modification: it is changed by the fact that in man the ego has been incorporated.
The same is the case with all other organs. And for the occurrences within the organs, especially in cases of disease, the permeation by the ego is of much greater importance than what happens in the animal’s organs, where there is no such permeation. This essential difference is still far too little regarded and men persist in off-hand pronouncements of this sort: “here I have a knife; well, a knife’s a knife, isn’t it? One knife is the same as another, so both, being knives, must have the same origin.” But suppose that one of these “identical” knives is a table knife, the other a razor. In that case the simple proposition that “a knife’s a knife” becomes untenable.
It is making the same mistake to explain the human eye and the animal eye by the same methods and terms. It is simply nonsense to seek for the explanation of anything in its mere external aspect; moreover such an approach is entirely barren as a foundation for study. Study founded on animal “material” simply hinders the adequate study of certain conditions in mankind; for it is only possible to form a just estimate of the dissimilarity here, by realising that in man it is precisely the peripheral organs which are the most permeated by the ego and moulded by it.
Source: Rudolf Steiner – GA 312 – Spiritual Science and Medicine – Lecture XIV – Dornach, April 3, 1920