Two Interpretations of Two Stoic Conditionals

Four different conditionals were known to the Stoics. The so-called 'first' (Philonian) conditional has been interpreted fairly uncontroversially as an ancient counterpart to the material conditional of modern logic; the 'fourth' conditional is obscure, and seemingly of little historical interest, as it was probably not held widely by any group in antiquity. The 'second' (Diodorean) and 'third' (Chrysippean) conditionals, on the other hand, pose challenging interpretive questions, raising in the process issues in philosophical logic that are as relevant today as they were then. This paper is a critical survey of some modern answers to four of the most tantalizing of these questions; the issues that I will discuss arise out of interpretations of the Diodorean and Chrysippean conditionals as expressions of natural law, and as strict implications. I will reject these interpretations, concluding with my own proposal for where they should be located on a 'ladder' of logical strength. The following passage from Sextus will form the basis of my discussion (from Outlines of Pyrrhonism [Pyrrhoneae Hypotyposes], as presented by Long and Sedley 1987b, 211). He has just introduced Philo's account of " a sound conditional " —by which I understand a true conditional—with the example " when it is day and I am talking, 'If it is day, I am talking' ". He then continues: