But the question as to the nature of the εἶδος became a vital question in every system of philosophy. Granting, for instance, that women in every clime and country formed one species, it was soon asked what constituted a species? If all women shared a common form, what was that form? Where was it? So long as it was supposed that all women descended from Eve, the difficulty might be slurred over by the name of heredity. But the more thoughtful would ask even then how it was that, while all individual women came and went and vanished, the form in which they were cast remained the same?
Here you see how philosophical mythology springs up. The very question what εἶδος or species or form was, and where these things were kept, changed those words from predicates into subjects. Εἶδος was conceived as something independent and substantial, something within or above the individuals participating in it, something unchangeable and eternal. Soon there arose as many εἴδη or forms or types as there were general concepts. They were considered the only true realities of which the phenomenal world is only as a shadow that soon passeth away. Here we have, in fact, the origin of Plato's ideas, and of the various systems of idealism which followed his lead, while the opposite opinions that ideas have no independent existence, and that the one is nowhere found except in the many (τὸ ἕν παρὰ τὰ πολλά), was strenuously defended by Aristotle and his followers.[17]
The same red thread runs through the whole philosophy of the Middle Ages. Men were cited before councils and condemned as heretics because they declared that _animal_, _man_, or _woman_ were mere names, and that they could not bring themselves to believe in an ideal animal, an ideal man, an ideal woman as the invisible, supernatural, or metaphysical types of the ordinary animal, the individual man, the single woman. Those philosophers, called _Nominalists_, in opposition to the _Realists_, declared that all general terms were _names only_, and that nothing could claim reality but the individual.
We cannot follow this controversy further, as it turns up again between Locke and Leibniz, between Herbart and Hegel. Suffice it to say that the knot, as it was tied by language, can be untied by the science of language alone, which teaches us that there is and can be no such thing as "a name only." That phrase ought to be banished from all works on philosophy. A name is and always has been the subjective side of our knowledge, but that subjective side is as impossible without an objective side as a key is without a lock. It is useless to ask which of the two is the more real, for they are real only by being, not two, but one. Realism is as one-sided as Nominalism. But there is a higher Nominalism, which might better be called the Science of Language, and which teaches us that, apart from sensuous perception, all human knowledge is by names and by names only, and that the object of names is always the general.