Logical Investigations (fragments)
Chrysippus's 3rd-century BCE foundational works on Stoic propositional logic — the five indemonstrable argument forms and the theory of the lekton
Tradition: Stoic logic / Hellenistic philosophy
The five indemonstrable argument forms and the foundations of propositional logic
Chrysippus's logical works (of which Diogenes Laertius catalogues over 300 titles on logic alone) survive only in fragments and reports preserved by Diogenes Laertius, Sextus Empiricus, Galen, Alexander of Aphrodisias, and others. The central achievement is propositional logic based on the five indemonstrable argument forms: (1) modus ponens, (2) modus tollens, (3) the denial of a conjunction, (4) exclusive disjunction with one disjunct affirmed, (5) inclusive disjunction with one disjunct denied. Chrysippus also developed the theory of the lekton (the "sayable" or propositional content), which is an incorporeal, providing a sophisticated theory of meaning. These logical innovations were largely forgotten until rediscovered by modern logicians, who recognised Chrysippean logic as a genuine anticipation of propositional calculus.
Author
Editions cited
- Fragments collected in SVF II (von Arnim), Logik; discussed in A.A. Long and D.N. Sedley, The Hellenistic Philosophers (Cambridge UP, 1987), vol. 1, chs. 31, 34-38; Susanne Bobzien, "Chrysippus and the Epistemic Theory of Vagueness" (Proceedings of the Aristotelian Society, 2002); Benson Mates, Stoic Logic (Berkeley, 1953; 2nd edn. 1961)
School Embodiments
Foundational Stoic logical system; Chrysippus's logic defined the Stoic organon.
"The five indemonstrables: (1) If p then q; p; therefore q." (Diogenes Laertius VII.80-81)
The five indemonstrable argument forms are a genuine anticipation of propositional calculus.
"Chrysippus's five indemonstrables correspond to the basic inference rules of modern propositional logic." (Benson Mates, Stoic Logic)
Purely rational analysis of the structure of valid inference.
"Chrysippus held that only what is demonstrable from the indemonstrables is valid." (Sextus Empiricus)
Rediscovered by modern analytic logicians as a genuine anticipation of truth-functional logic.
"Stoic logic, especially Chrysippus's, is a genuine propositional logic." (Benson Mates)
The theory of the lekton (sayable) provides a sophisticated account of propositional content.
"The lekton is what is signified — an incorporeal, neither body nor void." (Sextus Empiricus)
Realist about logical structure: valid inference tracks objective truth.
"A sound argument leads from true premises to a true conclusion." (Chrysippus, via Sextus)
Internal Tensions
The incorporeal status of the lekton sits uneasily with Stoic corporeal ontology; the relationship between Stoic logic and Stoic physics remains a subject of scholarly debate.
I. Time
Stoic cyclical cosmic time; logical truths hold across conflagrations.
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II. Space
The Stoic finite cosmos; logical structure is independent of spatial location.
Attributes
III. Matter
Stoic corporeal ontology; the lekton (sayable) is notably incorporeal — a rare exception.
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IV. Observer
The rational agent who grasps propositions and follows valid inferences.
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V. Energy
Stoic pneuma and conflagration cycle.
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VI. Information
Discrete propositions (lekta) as the bearers of logical content; foundational for information-theoretic semantics.
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Personas with the nearest attribute fingerprint
Historical figures whose own classification on the same six-dimensional grid lands closest to this work's. Computed by attribute-agreement on coordinates both address.
Computed school proximity
The work's attribute fingerprint scored against all schools using the same quiz scorer. Useful as a sanity check on the hand-curated embodiments above.
How Logical Investigations (fragments) resolves each dilemma
51 resolved positions across 4 dimensions, including 15 distinctive where the majority of schools go the other way · 6 unaligned.
Each dimension is sorted so minority positions come first. Mainstream positions are folded into an expandable list.
Time · 9 dilemmas · 5 distinctive
Persistence, the future, and the direction of becoming.
4 mainstream positions
Matter · 7 dilemmas, all mainstream
Observer · 37 dilemmas · 5 distinctive
Mind, agency, and the knower's relation to the known.