The Organon, or Logical Treatises, of Aristotle

What are we doing when we say of a man that he is an animal, that he is pale, that he is in the marketplace? The three answers differ in kind, and telling them apart decides what counts as a thing at all.

The Categories sorts whatever can be said of a subject into ten kinds: what it is, how much, of what sort, related to what, where, when, and four more. Underneath stands the individual, this man or this horse, which Aristotle calls primary substance; species and genus are substance only in a second sense, and if no individuals existed nothing else could. One mark sets substance apart: it stays one and the same while taking on opposites, pale then dark, good then bad.

• predication
• classification
• genus and species
• genus
• accident
• properties
• contraries
• substance
• categories
• quality
• quantity
• relation
• privation

Only a sentence that affirms or denies something can be true or false. Does that hold of a sentence about tomorrow, and if it does, is tomorrow already decided?

On Interpretation starts from words as signs fixed by agreement and narrows to the one kind of sentence logic needs, the statement; a prayer is a sentence too, but neither true nor false. Every affirmation has exactly one denial, and of such a pair one side must be true. Aristotle tests the rule on a sea battle that may or may not be fought tomorrow. Were one side true already, everything would happen of necessity and deliberating would be pointless, so for him neither is yet settled as true.

• language
• negation
• contradiction
• contingency
• necessity
• affirmation and negation
• truth

Which arguments compel their conclusion whatever they happen to be about? To find out, Aristotle puts letters where the things used to stand.

A syllogism, the Prior Analytics says, is speech in which, certain things being laid down, something different follows of necessity. Aristotle writes the terms as letters, sorts the pairs of premises into three figures and asks of each whether a conclusion follows. A few patterns of the first figure are plain on their face, and he calls them perfect; the others are proved by leading them back to these. A pattern that compels nothing he exposes with terms, swan and snow among them, that make its premises true and its conclusion false.

• syllogistic logic
• syllogism
• logic
• inference
• conversion
• modal logic

A valid argument from true premises may still explain nothing. What must be added before a proof gives knowledge, and where does the chain of proofs begin?

To know a thing in the strict sense, the Posterior Analytics holds, is to know its cause and that it could not be otherwise. A demonstration is thus a syllogism whose premises are true, first, and prior to the conclusion as its causes. That the planets are near can be inferred from their not twinkling, but only the reverse order explains. To those who say proof must run forever or in a circle, Aristotle answers that first principles are not proved: the mind grasps them, starting from perception, memory and experience.

• demonstration
• knowledge
• causation
• essence
• universals

Most arguing happens where no proof is to be had, between a questioner and someone defending a view. How is that done well, and how does one see through a refutation that only looks like one?

Dialectic, in the Topics, reasons from what everyone, or most people, or the wise accept. Aristotle names three uses: training, conversation and philosophy, since the first principles of a science cannot be proved inside that science. Most of the treatise supplies ways to test a proposed definition, which is easier to overturn than to establish. The Sophistical Refutations turns to the counterfeit: thirteen ways an argument can seem to refute, six that hang on language, such as one word with two meanings, and seven that do not.

• dialectic
• argumentation
• definition
• fallacies
• ambiguity
• equivocation
• refutation