Their type of reasoning was--
If A, then B
But A
B
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 [Footnote: Here we may recall the warning
of Arago to call nothing impossible outside the range of pure
mathematics] 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.
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