The rules for valid reasoning laid down by the Greeks were intended primarily for use in politics, but in politics reasoning has in fact proved to be more difficult and less successful than in the physical sciences.

The chief cause of this is to be found in the character of its material.

We have to select or create entities to reason about, just as we select or create entities to stimulate our impulses and non-rational inferences.

In the physical sciences these selected entities are of two types, either concrete things made exactly alike, or abstracted qualities in respect of which things otherwise unlike can be exactly compared.

In politics, entities of the first type cannot be created, and political philosophers have constantly sought for some simple entity of the second type, some fact or quality, which may serve as an exact 'standard' for political calculation.