Prior AnalyticsAristotle
About Prior Analytics
The is where Aristotle invents formal logic. The work sets out the theory of the syllogism: a deductive argument in which a conclusion follows necessarily from two premises by virtue of their logical form alone.
Book I defines the syllogism and systematically enumerates its valid figures and moods. A syllogism consists of three terms (major, middle, minor) arranged in two premises and a conclusion. Aristotle identifies three figures, distinguished by the position of the middle term, and catalogs which combinations of universal and particular, affirmative and negative premises yield valid conclusions. He proves completeness by showing that all valid syllogisms in the second and third figures can be reduced to syllogisms in the first figure, which he takes as self-evident.
Book II treats modal syllogisms (involving necessity and possibility) and extends the analysis to arguments with mixed modal and assertoric premises. This section is notoriously difficult, and Aristotle's modal logic has generated disagreement from Theophrastus to the present.
The is not a treatise on how people do reason but on how valid inference works. Aristotle distinguishes it from the , which concerns demonstration (syllogisms from true and necessary premises), and from the , which concerns dialectical reasoning from probable premises. The syllogistic remained the dominant framework for logic until Frege, and its influence on the structure of scientific and philosophical argument is impossible to overstate.