How did Aristotle use deductive reasoning to study philosophical questions?

Aristotle’s use of deductive reasoning is primarily defined by his development of the syllogism, a systematic method of inference detailed in his work, the Prior Analytics.12

Rather than viewing logic as a theoretical science, Aristotle treated it as a "tool" (Organon) applicable to all fields of inquiry. His deductive system functions by deriving a conclusion from two premises. A classic example is: "Every Greek is human. Every human is mortal. Therefore, every Greek is mortal".21

Key aspects of his approach include:

• Term Logic: Aristotle focused on the logical relations between "terms" (subjects and predicates). He introduced the use of schematic variables (such as A, B, and C) to represent these terms, allowing him to identify valid patterns of argument regardless of the specific content.12
• Validity: He sought to determine which "moods" of syllogisms were valid, establishing rules to ensure that true premises could never lead to a false conclusion.1
• Psychological Necessity: Aristotle believed that when premises form a "single opinion," the mind is psychologically compelled to accept the logical conclusion.2

While his syllogistic method is a foundational achievement in logic, it represents only a fragment of logical reasoning, as it does not account for all types of inferences.13