Arguments are—normally—compounds of assertibles. They are defined as a system of at least two premises and a conclusion (D. L. 7.45). Syntactically, every premise but the first is introduced by ‘now’ or ‘but’, and the conclusion by ‘therefore’. An argument is valid if the (Chrysippean) conditional formed with the conjunction of its premises as antecedent and its conclusion as consequent is correct (S. E. PH 2.137; D. L. 7.77). An argument is ‘sound’ (literally: ‘true’), when in addition to being