Aristotle's method, in ethics as in all other fields, was critical and empirical. In the study of any subject he began by collecting, analyzing, and grouping all relevant facts in order to determine their meaning and relations with each other, and this gave him a systematic and factually correct basis from which to generalize about underlying rules or principles. In generalizing, he used either the inductive approach, reasoning from many observed single instances to a universal proposition, or the syllogism, a means of deductive reasoning which he invented, and defined as "certain things being stated, something else follows of necessity without need of further testimony," i.e., proceeding from previously established general rules or facts down to particular instances.
The syllogism is used frequently by Aristotle in analytical sections of the Nicomachean Ethics. It has two premises — one major (universal) and the other minor (particular), and in its simplest form works as follows:
Major Premise: All A is B or: All men are mortal.
Minor Premise: C is part of A: Socrates was a man.
Conclusion: C is B: Socrates was mortal.
Of course, as Aristotle frequently warned, it is possible to reason correctly from false premises, thus coming up with a logically correct but untrue conclusion, and therefore it is of essential importance to make certain that the premises of a syllogism are true and sufficiently comprehensive to cover all cases.
These modes of reasoning illustrate the most significant difference between the Platonic and Aristotelian systems. Plato postulated the existence of ideal, absolute standards and forms, against which all human things had to be measured. Aristotle, while not specifically denying the existence of these abstract standards, approached the same questions from another direction, and tried to determine the nature of things by empirical observation and logical analysis, never stating a hypothesis without first testing it against the data.
Aristotle's work and method have had an unparalleled influence on the development of thought. In the Middle Ages he was considered an absolute authority on nearly every subject, referred to by Saint Thomas Aquinas as "The Philosopher" and by Dante as "Master of those that know," although much of Aristotle's insistence on empirical method was ignored by his medieval disciples.
Aristotle's technique and influence continued to play a large role in the post-medieval world, and he is considered by many as the father of research and empirical science, and the founder of such diverse disciplines as logic, psychology, political science, literary criticism, scientific grammar, physics, physiology, biology and most other natural sciences. Some scholars, in fact, have described the intellectual history of western civilization as a permanent debate in which Aristotle has sometimes triumphed and sometimes not, but in which at all times his spirit and principles have acted as the substructure and inspiration of progress.