Ancient Athens buzzed with arguments. Aristotle turned the noise into a toolkit: logic to test reasons, and categories to sort what we mean when we say that something is.

WHY LOGIC?

For Aristotle, logic is the grammar of thought. We build arguments from three layers: terms (names like 'human' or 'mortal'), propositions (claims that join terms, such as 'All humans are mortal'), and syllogisms (two propositions leading to a conclusion). Logic checks validity—the form that guarantees the conclusion if the premises are true—before we worry about truth.

“All human beings by nature desire to know.”

— Aristotle, Metaphysics I.1

THE SYLLOGISM: THREE-STEP REASONING

A classic syllogism links ideas through a middle term: All mammals are warm-blooded; all whales are mammals; therefore, all whales are warm-blooded. 'Mammals' is the middle term that connects whales to warm-bloodedness. The beauty is mechanical: valid form makes the reasoning watertight even before we fact-check the premises.

💡 Pro Tip

When assessing an argument, find the middle term and test two things: are the premises true, and is the form valid? Watch for equivocation—when the same word shifts meaning between premises.

TEN CATEGORIES: HOW WE SAY 'IS'

Aristotle's Categories maps the basic kinds of predicates we can assert of a thing. The ten are: substance (this horse), quantity (two meters), quality (brown, virtuous), relation (larger than), place (in Athens), time (yesterday), position (sitting), state or having (armed, shod), action (cutting), and passion or being-acted-on (being cut). Think of them as ten camera angles for describing reality—not boxes of objects, but ways of speaking about what is.

WHAT REALLY IS VS HOW IT CAN BE
Substance
  • Primary bearer of predicates; exists in its own right.
  • Persists through change.
  • Example: Socrates, this oak tree.
Accidents
  • Depend on a substance; cannot exist on their own.
  • Can change without destroying the thing.
  • Examples: being tan, sitting, two meters tall.

CLASSIFY TO KNOW

Logic also powers definition. We identify a thing by genus and differentia—its broader class and the feature that sets it apart: 'Human is a rational animal.' Good science, Aristotle says in the Posterior Analytics, builds demonstrations from such definitions, so that knowing why something is follows from knowing what it is.

“To say of what is that it is, and of what is not that it is not, is true.”

— Aristotle, Metaphysics 1011b
Key Takeaways
  • Logic gives form to reasoning: terms, propositions, syllogisms.
  • Validity is about structure; truth is about premises.
  • The ten categories are basic kinds of predication, not bins of objects.
  • Substance underlies accidents; accidents describe but do not constitute a thing.
  • Definitions by genus and differentia ground demonstration and scientific knowledge.