Sextus Empiricus Pyrrhoniae Hypotyposes 2.12.139

[139] For an argument such as this—‟If it is night, it is dark; but in fact it is night; therefore it is dark”—is indeed conclusive, since the syllogism ‟it is night, and if it is night it is dark, therefore it is dark” is a valid one, but, when it is day-time, it is not true. For the antecedent combination—‟it is night, and if it is night it is dark”—is false since it contains the falsehood ‟it is night;” for the combination which contains a falsehood is false. Hence they also say that a true argument is that which deduces a true conclusion from true premisses.