Man has therefore to create entities that shall be the material of his reasoning, just as he creates entities to be the objects of his emotions and the stimulus of his instinctive inferences.
Exact reasoning requires exact comparison, and in the desert or the forest there were few things which our ancestors could compare exactly.
The heavenly bodies seem, indeed, to have been the first objects of consciously exact reasoning, because they were so distant that nothing could be known of them except position and movement, and their position and movement could be exactly compared from night to night.
In the same way the foundation of the terrestrial sciences came from two discoveries, first, that it was possible to abstract single qualities, such as position and movement, in all things however unlike, from the other qualities of those things and to compare them exactly; and secondly, that it was possible artificially to create actual uniformities for the purpose of comparison, to make, that is to say, out of unlike things, things so like that valid inferences could be drawn as to their behaviour under like circumstances.

Geometry, for instance, came into the service of man when it was consciously realised that all units of land and water were exactly alike in so far as they were extended surfaces.

Metallurgy, on the other hand, only became a science when men could actually take two pieces of copper ore, unlike in shape and appearance and chemical constitution, and extract from them two pieces of copper so nearly alike that they would give the same results when treated in the same way.
This second power over his material the student of politics can never possess.

He can never create an artificial uniformity in man.

He cannot, after twenty generations of education or breeding render even two human beings sufficiently like each other for him to prophesy with any approach to certainty that they will behave alike under like circumstances.
How far has he the first power?
How far can he abstract from the facts of man's state qualities in respect of which men are sufficiently comparable to allow of valid political reasoning?
On April 5th, 1788, a year before the taking of the Bastille John Adams, then American Ambassador to England, and afterwards President of the United States, wrote to a friend describing the 'fermentation upon the subject of government' throughout Europe.