A Guide to Stoicism 3 ¶20–21 — St. George Stock (modern guide)

St. George Stock (modern guide), A Guide to Stoicism 3 ¶20–21

The important part played by conjunctive propositions in their logic led the Stoics to formulate the following rule with regard to the material quality of such propositions: Truth can only be followed by truth, but falsehood may be followed by falsehood or truth. Thus if it be truly stated that it is day, any consequence of that statement, e.g. that it is light, must be true also. But a false statement may lead either way. For instance, if it be falsely stated that it is night then the consequence that it is dark is false also. But if we say, "The earth flies," which was regarded as not only false but impossible this involves the true consequence that the earth is. Though the simple syllogism is not alluded to in the sketch which Diogenes Laertius gives of the Stoic logic, it is of frequent occurrence in the accounts left us of their arguments. Take for instance the syllogism wherewith Zeno advocated the cause of temperance-- One does not commit a secret to a man who is drunk. One does commit a secret to a good man. A good man will not get drunk.