thema (plural: ta), in Stoic logic, a ground rule used to reduce argument forms to basic forms. The Stoics analyzed arguments by their form (schema, or tropos). They represented forms using numbers to represent claims; for example, ‘if the first, the second; but the first; therefore the second’. Some forms were undemonstrable; others were reduced to the undemonstrable argument forms by ground rules (themata); e.g., if R follows from P & Q, -Q follows from P & -R. The five undemonstrable arguments are: (1) modus ponens; (2) modus tollens; (3) not both (P and Q), P, so not-Q; (4) P or Q but not both, P, so not-Q; and (5) disjunctive syllogism. The evidence about the four ground rules is incomplete, but a sound and consistent system for propositional logic can be developed that is consistent with the evidence we have. (See Diogenes Laertius, Lives of the Philosophers, 776–81, for an introduction to the Stoic theory of arguments; other evidence is more scattered.) See also DOXOGRAPHERS , FORMAL LOGIC , LOGICAL FORM , STOICIS. H.A.I.