LOGIC AND THE TENSES

Press, Volume XCVII, Issue 28512, 15 February 1958, Page 3

LOGIC AND THE TENSES

yime and Modality. By A. N. Prior. Clarendon Press: Oxford University Press. viii 148 PP-

We often think of logic as a dry and rigid system: we feel that our thought and experience, the w orld as we know it and live in it and talk about it, is cramped gnd distorted if it is forced into the logician’s mould. Some philosophers, taking this view, say that should stick to ordinary language, appreciate its subtlety and complexity, and defend it from the restraints of formal logic. This may be a fair comment on elementary traditional logic, but it does*not apply to logic as a whole, as we can see from this book in which Professor A. N. prior, of the University of Canterbury, adapts and develops logic tg-make it more adequate to our experience of time. Prior, a New Zealander with an international reputation as a logician, was invited to deliver the John Locke lectures for 1955-58 at the University of Oxford, which are published (with some additional material) in this book on “Time and Modality.” His purpose is to do justice to such statements as “It is summer ln ; England,” “Professor Carnap will be flying to the moon,” “I was eating chocolates last May”— statements for -whose meaning it is vitally important that they are in the present or the future or the past tense, and which can be

true at one time and false at another, whereas each of the timeless “propositions” of ordinary logic is either eternally true •or eternally false. Indeed, Professor Prior tells us, these tensed statements do even odder things than oscillating between truth and falsehood: although it is true now that he is a logician, it was not true in 1850 that he was going to be a logician, because he did not exist in 1850 and there were therefore no facts about him at all. And he would say that although it is true now that a war broke out in 1939, it was not true in, say, 1930 that a war was going to break out nine years later, because in 1930 the outbreak of that war was as yet undetermined, it was the product of human decisions made after that date.

It is clear that the logic of tensed statements is far from simple, but that does not make it unsystematic. Professor Prior takes various system of symbolic logic—in particular several that have been developed in the last 30 years to cope with “modality,”' that is with the notions of necessity and possibility—and applies these ,to statements involving time. Just as a physicist may apply to some set of actual physical phenomena a calculus that has been developed by a pure mathematician with no thought of

any practical application, so Prolessor Prior rummages through the stock of logical calculi built UP by the symbolic logicians to nnd ones which (perhaps with some alterations) will fit the facts about time.

Professor Prior has both the interest of the formal logician in constructing systems and seeing what they will do, and the interest of the metaphysician in giving a true picture of the world, taking account of the reality of time and the indeterminacy of the future. But the reader may suspect that the first interest sometimes carries him away, and he plunges .into systematic complexities which, though fascinating in themselves, are not really necessary for his metaphysical purpose. Attempts to prove determinism from traditional logic alone have always failed, and even if you are perverse enough to be an indeterminist you are not thereby constrained from usiqg a logic of the traditional sort.

This is a book of importance for philosophy in general as well as for logic, but it must be admitted that while the general reader can appreciate the theme and purpose, to follow the details of the discussion one would require a good knowledge of recent symbolic logic. But since Professor Prior is a historian of philosophy as well as a logician, there’is an interesting and readable appendix which traces the history of the attitudes of logicians to times and tenses.