A general idea is one that concerns multiple objects, and generalization is the means by which such ideas are obtained.

Generalization involves the convergence of two processes – comparison and abstraction. We compare several objects to see what they have in common, and then we abstract. The common qualities form the basis for the general idea – for example, we might compare different men to see what they have in common and, from this, abstract and formulate the general idea that all men have these qualities (sensory abilities, intelligence, activity, etc.).

What's the value of such ideas? In the Middle Ages, some philosophers – called realists – maintained that general ideas correspond to real things, while others insisted that general ideas are entirely subjective and that the term “general” is merely a flatus vocis – it doesn't refer to anything real or concrete. But for language, they believed, there wouldn't be any general ideas at all – a position known as nominalism.

Condillac and Taine were among the proponents of nominalism, while realism dates back to antiquity. Platonic ideas aren't general types in any strict sense, but they were conceived as applying to all individuals and thus can be considered, at least in part, as substantive types. Plato, in short, was a realist.

But experience alone shows the absurdity of realism. There's no such thing as a general type in itself. The resemblances that exist between particular objects are entirely a function of their common source.

Yet neither can we accept pure nominalism.