Full Idea
Definitions are identical to universal propositions in meaning, and only differ in syntax, for whoever says 'Man is a rational mortal animal' says the same thing in meaning as whoever says 'If something is a man, that thing is a rational mortal animal'.
Gist of Idea
'Man is a rational mortal animal' is equivalent to 'if something is a man, that thing is a rational mortal animal'
Source
Sextus Empiricus (Against the Professors (six books) [c.180], 11.8)
Book Reference
'The Hellenistic Philosophers:Vol.1 translations', ed/tr. Long,A. /Sedley,D. [CUP 1987], p.180
A Reaction
How strikingly like Bertrand Russell's interest and solutions. Sextus shows a straightforward interest in logical form, of a kind we associate with the twentieth century. Did Sextus Empiricus invent quantification?