Diodorus Cronus

Life and Historical Context

Diodorus Cronus was a Hellenistic logician and dialectician associated with the Megarian school of philosophy. He was probably born in Iasus, a city in Caria (Asia Minor), in the late 4th century BCE and was active in the generation following Aristotle. His epithet “Cronus” may have been a family name or a nickname; ancient sources disagree on its origin.

Very little is known about his life with certainty, and almost all information about him comes from later authors such as Cicero, Sextus Empiricus, and Epictetus. These testimonies portray Diodorus as a highly skilled dialectician, renowned for devising and solving sophisticated logical puzzles. He is often linked with other Megarians—such as Eubulides of Miletus—who collectively helped to shape early logical theory through paradoxes and argumentative challenges.

Ancient reports suggest that Diodorus spent at least part of his career at the Hellenistic courts (possibly at the court of Ptolemy in Alexandria), engaging in public debates. He is remembered as the teacher of the influential Stoic philosopher Zeno of Citium, indicating that his ideas formed a bridge between Megarian logic and early Stoic logic. However, no works of Diodorus have survived; what is known of his doctrines is reconstructed from fragments, paraphrases, and critical discussions in later philosophical and rhetorical literature.

The Master Argument and Modal Logic

Diodorus is best known for the so‑called Master Argument (ho kyrios logos), a celebrated piece of reasoning in ancient discussions of modality—that is, of possibility, necessity, and contingency. The argument is reported most fully by later sources and appears to have been designed to defend a strictly deterministic view of what is possible.

The Master Argument hinges on the tension between three theses:

1. Every past truth is now necessary (the fixity of the past).
2. The impossible does not follow from the possible (a principle of logical consistency).
3. There is something that is possible which neither is nor will be true (a robust sense of possibility beyond what actually occurs).

Diodorus argued that these three claims are jointly inconsistent. Ancient reports suggest that he took the first two theses as intuitively compelling and concluded that the third must be rejected. From this rejection he inferred his influential definition:

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The possible is that which either is true or will be true.

On this definition, any proposition that never attains truth (at any time) is not genuinely possible. What might appear as a merely possible but forever unactualized state of affairs is, from Diodorus’ standpoint, simply impossible. This view has been interpreted as a form of logical determinism: given the actual course of the world’s history, no alternative future is truly possible.

Later philosophers debated the plausibility of each premise. Some, especially among the Stoics, questioned Diodorus’ notion of necessity and time, developing alternative theories of modality that preserved a richer sense of unrealized possibilities. Others examined whether the principle that “the impossible does not follow from the possible” could be maintained without revision. The Master Argument thus became a focal point for ancient discussions of the logic of time, the status of future contingents, and the scope of divine or cosmic necessity.

Modern scholarship continues to disagree on the exact reconstruction of the argument’s steps and on whether Diodorus was primarily motivated by metaphysical determinism, by purely logical considerations, or by both.

Paradoxes and Logical Innovations

In addition to the Master Argument, Diodorus is associated with a range of logical paradoxes and conceptual innovations that contributed to early philosophy of language and metaphysics.

One famous puzzle is the “Growing Argument”, reported in various forms in antiquity. Though its precise structure is debated, it appears to involve questions about identity through change—for example, whether something that undergoes gradual addition or loss of parts remains the same object. This line of questioning anticipates later problems such as the Ship of Theseus, highlighting difficulties in defining persistence over time.

Diodorus is also credited with work on conditional propositions and implication. He reportedly defended a notion of a true conditional in which the antecedent could never be true while the consequent was false across all times. This approach, sometimes called the Diodorean conditional, ties logical implication closely to the actual course of events and their temporal unfolding, rather than to purely formal patterns alone. Later Stoics developed their own theory of conditionals, partly in dialogue with Diodorus’ ideas.

Further, Diodorus contributed to discussions of mereology (the relations of parts and wholes) and change, asking whether a thing can become something it previously was not without ceasing to be what it was. These explorations sought to clarify the logical structure underlying ordinary claims about growth, transformation, and persistence.

Because none of Diodorus’ own writings survive, it is difficult to determine how systematically these diverse puzzles were integrated into a single philosophical system. Nonetheless, ancient testimonies consistently highlight his ingenuity in devising arguments that exposed hidden assumptions in common views about time, possibility, and identity.

Influence and Reception

Diodorus Cronus exerted a significant, if indirect, influence on later ancient philosophy. His most immediate impact appears in Stoic logic and metaphysics. As a teacher of Zeno of Citium, he helped shape the intellectual environment out of which Stoicism emerged. Stoic logicians such as Chrysippus engaged critically with Diodorus’ views on conditionals and modality, adopting some elements while rejecting others, especially his strict determinist definition of the possible.

In Hellenistic and Roman philosophy, Diodorus’ Master Argument became a standard reference point in debates over fatalism and future contingents. Authors like Cicero discussed his views as emblematic of one influential attempt to reconcile the fixity of the past with logical principles about possibility and necessity. Skeptical writers, including Sextus Empiricus, cited Diodorus both to illustrate the sophistication of dialectical argument and to show the deep disagreements among dogmatic philosophers.

Medieval and early modern thinkers encountered Diodorus largely through these secondary reports. While his name was less prominent than Aristotle’s, the central problem he posed—how to define possibility given a fixed history—resonated in subsequent discussions of divine foreknowledge, predestination, and modal logic. Modern formal logicians and historians of logic often return to Diodorus as an early figure grappling with issues that anticipate temporal logic, branching time models, and debates about actualism versus possibilism.

Contemporary assessments vary. Some scholars view Diodorus as primarily a clever dialectician whose arguments illuminate the conceptual tensions in common-sense talk about the future. Others regard him as an early proponent of a robust actualist or determinist metaphysics, in which the space of genuine possibilities is strictly limited to what occurs in the single actual history of the world. Because the evidence is fragmentary and filtered through polemical sources, no consensus interpretation has emerged.

Despite these uncertainties, Diodorus Cronus is widely recognized as a central figure in early logic, notable for formulating one of antiquity’s most influential arguments about modality and for posing puzzles that continued to challenge philosophers well beyond the Hellenistic age.