(Chapter IV — The Material of Political Reasoning, Page 114)
But men can and do reason, though reasoning is only one of their mental processes. 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. This search has hitherto been unsuccessful, and the analogy of the biological sciences suggests that politicians are most likely to acquire the power of valid reasoning when they, like doctors, avoid the over-simplification of their material, and aim at using in their reasoning as many facts as possible about the human type, its individual variations, and its environment. Biologists have shown that large numbers of facts as to individual variations within any type can be remembered if they are arranged as continuous curves rather than as uniform rules or arbitrary exceptions. On the other hand, any attempt to arrange the facts of environment with the same approach to continuity as is possible with the facts of human nature is likely to result in error. The study of history cannot be assimilated to that of biology.