True it is that the greater part of the words and ideas in ordinary
use are in the present state of our knowledge not capable of being
defined at all in this sense. The definition of any " natural kind,"
to use J. S. Mill's term, would have to give us not only the specific
characteristics of the kind in question, but their " immediate
cause," in other words, to show how they come to be what they
are : and it is barely conceivable that in this region of natural
history we shall ever get beyond more or less probable conjecture.
We have verae causae in natural selection, adaptation, and the like,
but to assign the proxima causa of any existing type we should
have to know exactly in what manner and proportions their effects
took place in the particular instance. There is no reason to sup-
pose, however, that Spinoza would have been afraid of saying that
very few perfect definitions had been found or could be expected.
