Socrates and Plato may have invented reason and rationalism, but the man who turned reason into a powerful tool was Aristotle. He did so by a single act: the creation of the syllogism.
Humanity had understood causal relationships for at least 100,000 years before Aristotle. Hammurabi had codified causal relationships in his laws 1400 years before Aristotle. But Aristotle moved one step higher in abstraction -- he, in effect, tackled the problem of the causality of causality.
We all know of Aristotle's syllogism from the classic textbook example: "All men are mortal; Socrates is a man; therefore, Socrates is mortal". Whoop-de-doo; it doesn't look very earth-shaking, does it? The big trick lies in the fact that the syllogism provides us with a rock-solid means of assembling sequences of verbal reasoning. The basic form of the syllogism might be expressed in this schematic form:
A relates in this manner to B.
B relates in that manner to C.
Therefore A relates in some third manner to C.
This requires that all of the "relates in this or that manner" relationships be transitive as a group. For example, identity is a transitive relationship: if "A is B" and "B is C", then "A is C". Simple relationships of magnitude are also transitive: if "A is greater than B" and "B is greater than C" then "A is greater than C". Some relationships are not transitive: if "A hates B" and "B hates C", then we cannot conclude that "A hates C". But there are plenty of relationships that are transitive, and so the syllogism remains a powerful tool.
It's true that many earlier civilizations had toyed with sequences of statements arranged in such a manner as to suggest a conclusion, but these were not syllogistic arguments in the sense that we mean. For example, an ancient Chinese document declares:
“If terms are not correct, speech will not go smoothly.
If speech does not go smoothly, things will not succeed.
If things do not succeed, rites and music will not flourish.
If rites and music do not flourish, corporal and other punishments will not hit the mark.
If corporal and other punishments do not hit the mark, the people will have no place to set hand or foot.”
Structurally, this looks like a syllogism, but the causalities it claims are not convincing. This has none of the tightness of the Aristotelian syllogism. It may be reasoning, but it is not logic.
The syllogism provided thinkers with a rock-solid means of putting together propositions. In the hands of Socrates, reason was often convincing, but it was always possible to challenge the connections he made between the various propositions he used.
Aristotle provided us with a simple formula for assembling sequences of any length. All you had to do was put everything into the form of proper syllogisms, and then somehow get syllogisms that connected with each other. Then you could proceed from one initial proposition as far as you wished to get to your conclusion. The length of the sequence didn't matter; so long as the syllogisms were valid, the conclusion was the inevitable result of the proposition.