Reason

Reason is the ability to think in terms of abstractions, and to use those abstractions to arrive at logically sound conclusions.

The most influential invention since the invention of agriculture in 8,000 BC was the discovery of formal logic by Aristotle in 350 BC. People could now reason correctly for the first time. Previously reasoning had been based on intuition and experience.

It was Aristotle's Prior Analytics that "marked the invention of logic as a formal discipline in that the work contains the first virtually complete system of logical inference. ... A syllogism consists of three different categorical statements: two premises and a conclusion. The Prior Analytics tells us which pairs of categoricals logically yield a third." This is the very heart of logic. Furthermore, Aristotle's "Posterior Analytics extends syllogistic [reasoning] to science and scientific explanation. A science is a deductively ordered body of knowledge about a definite genus or domain of nature. Scientific knowledge consists not in knowing that, for example, there is thunder in the clouds, but rather in knowing why there is thunder in the clouds. So the theory of scientific knowledge is a theory of explanation and the vehicle of explanation is the syllogism." (1)

A classic example of a logical syllogism is: All men are mortal. Socrates is a man. Therefore Socrates is mortal.

But although humanity could now reason correctly when it wanted to, it still lacked the ability to prove that it had reasoned correctly, since much of reasoning depends on the use of logical analogy. The world had to wait another 2,000 years for the second and final piece of the puzzle to appear, which was the Scientific Method. Only then was mankind's greatest tool, reason, complete.

But with power comes responsibility.

(1) The material on Aristotle is from The Cambridge Dictionary of Philosophy, edited by Robert Audi, 1995, page 40.