[222] According to Sext. Pyrrh. ii. 100, Math. viii. 143 and 156, the Stoics distinguished between σημεῖα ὑπομνηστικὰ and σημεῖα ἐνδεικτικά. The definition of the latter was ἐνδεικτικὸν ἀξίωμα ἐν ὑγιεῖ συνημμένῳ καθηγούμενον (or προκαθηγούμενον) ἐκκαλυπτικὸν τοῦ λήγοντος· the ὑγιὲς συνημμένον was a sentence with both the leading and inferential clauses true. Sext. Pyrrh. ii. 101; 106; 115; Math. viii. 249. [223] Diodorus had said that Only what is, or what will be, is possible. The Stoics, and in particular Chrysippus, define δυνατὸν as what is capable of being true (τὸ ἐπιδεκτικὸν τοῦ ἀληθὲς εἶναι), if circumstances do not prevent; ἀδύνατον as ὃ μή ἐστιν ἐπιδεκτικὸν τοῦ ἀληθὲς εἶναι. From the δυνατὸν they distinguish the οὐκ ἀναγκαῖον, which is defined as ὃ καὶ ἀληθές ἐστι καὶ ψεῦδος οἷόν τε εἶναι τῶν ἐκτὸς μηδὲν ἐναντιουμένων (Plut. Sto. Rep. 46, p. 1055; Diog. 76; Boëth. De Interp. 374, Bas. The same thing is also stated in Alex. Aphr. De Fato, c. 10, p. 30. δυνατὸν εἶναι γενέσθαι τοῦτο δ’ ὑπ’ οὐδενὸς κωλύετα γενέσθαι κἂν μὴ γένηται.) On the other hand, ἀναγκαῖον is, what is both true and incapable of being false, either in itself or owing to other circumstances. Diog. and Boëth. There was probably another definition of οὐκ ἀναγκαῖον, as ὃ ψεῦδος οἷόν τε εἶναι τῶν ἐκτὸς μὴ ἐναντιουμένων· so that it might be said (Boëth. 429) that the οὐκ ἀναγκαῖον was partly possible and partly impossible, without contradicting (as Boëth. and Prantl, p. 463, believe) their other statement, that the δυνατὸν was partly necessary and partly not necessary. The conceptions of the Possible and the Not-necessary are thus made to overlap, the former including the Necessary and Not-necessary, the latter the Possible and the Not-possible. To defend his definition of the Possible against the κυριεύων of Diodorus, Chrysippus denied the statement, δυνατῷ ἀδύνατον μὴ ἀκολουθεῖν, without exposing the confusion contained in it between sequence in time and causal relation (Alex. Anal. Pr. 57, b; Philop. Anal. Pr. xlii. b; Schol. in Arist. 163, a; Cic. De Fato, 7, 13; Ep. ad Div. ix. 4). Cleanthes, Antipater, and Panthoides preferred to attack another leading clause of Diodorus, the clause that Every past occurrence must necessarily be true (Epictet. Diss. ii. 19, 2 and 5). The Aristotelian position in reference to a disjunction, that When the disjunction refers to something future, the disjunction itself is true, without either clause being necessarily true, was not accepted by the Stoics (Simpl. Cat. 103, β). [224] Plut. Sto. Rep. 46, p. 1055, justly insists on this point. [225] Prantl, pp. 467–496. [226] Diog. 45; Sext. Pyrrh. ii. 194, see above p. 65. [227] Both were included by the Peripatetics under the term hypothetical. In the same way the Stoics include both among the five ἀναπόδεικτοι. See below p. 119, 2. [228] Chain-argument seems to have been also treated of in the categorical form. See p. 120, 3.