The Nachlass of A.N. Prior
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From Arthur N. Prior to Benson Mates on 06/08/1954

This text has been transcribed by: Fabio Corpina, University of Cagliari, Italy [removed]

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Letter from Arthur N. Prior to Benson Mates, dated 6. Aug., 1954Dept. of Philosophy CANTERBURY UNIVERSITY COLLEGE CHRISTCHURCH, N.Z. 6/8/54 PS. It goes without saying that I've enjoyed & profited by your book immensely. A.P. Dear Professor Mates, I enclose a small paper provoked by your book on Stoic Logic (I've submitted it to a periodical for publication, but haven't yet heard about it). A couple of other points arising out of the same work:- (1) On p. 81 you say of the schema Cpp, CNpp, ApNp p that ‘no clue is offered on how this sort of argument would be analyzed into the five basic arguments’. But isn't just such a clue given in the paragraph from Sextus that you have at the top ofp. 106? This paragraph suggests that they had some such principle of composition as this:- If one of the members of a true disjunction yields a certain conclusion when combined with one premiss, and the other member yields the same conclusion when combined with another premiss, then the two premisses yield the conclusion. By this principle the argument in question is ‘reduced’ to two (disjunctively combined) applications of ponendo ponens.
{Page 2 of the letter} - It's of some interest that the post-Renaissance logicians (e.g. the Port Royalists & Watts) classified the dilemma along with the sorites as a species of ‘compound argument’. – Possibly the thing I've suggested was another of the 4 principles of reduction. (2) Regarding the fourth opinion about implication recorded in the extract from Sextus at top of p. 48 - Those who judge by “suggestion”’. It seems to me that there's something to be said for the 
Edited by Fabio Corpina. The original letter is kept in the Prior collection at Bodleian Library, Oxford, Box 4.
view that there were Peripatetics. The word that you translate in effect (its consequent is in effect included in its antecedent) is  \n, & it is noteworthy that in An. Pr. 67 b 3, 5 Aristotle uses the
complementary term  \r  in describing the difference between a man who has the premisses but hasn't drawn the conclusion & the man who has drawn it.
('The consequent is potentially 'The antecedent is potentially the consequent & the consequent is the
antecedent brought to actuality.
And the quotation you give from Alexander on pp. 125-6 about useless syllogismus reinforces the conjecture that the proponents of Sextus 4th opinion were Peripatetics. - Yours sincerely Arthur N. Prior •  University of Copenhagen •  Aalborg University •