New Zealand and England, 1914–1969

Arthur Prior

Tense is not a quirk of grammar to be translated away. Past, present and future are real, and logic should be built to handle them. When a headache ends, we are glad of something no dated fact can capture: that it is over.

The view

Arthur Norman Prior was born in New Zealand, taught at Canterbury University College in Christchurch, and later moved to Manchester and then Oxford. He invented tense logic, a formal system for reasoning with "it was the case that" and "it will be the case that", set out in Time and Modality (1957) and Past, Present and Future (1967).

Philosophically he was an A-theorist, and in later work a presentist: he held that only the present exists. The past is what was the case, the future what will be. Neither is a further region of reality lying somewhere along a line.

The argument

Tenses are operators. Take a present-tense sentence, "it is raining". Prior added operators that shift it in time. P: it has been the case that. F: it will be the case that. Their strong forms, H and G, say it has always been, and always will be, the case. These can be iterated and combined, and they obey laws that can be studied like the laws of any other logic. The truth of the sentence changes as time passes; that is not a defect but the point.

Tense cannot be eliminated. B-theorists say that "the headache is over" really means something like "the end of the headache is earlier than this utterance". Prior's reply, in a 1959 paper, is a single example.

Thank goodness that's over. Title of Prior's paper in Philosophy (1959)

When a pain ends, that is what we say, and we mean it. But we are not thankful that the end of the pain is earlier than some date, or earlier than our saying so: those facts were true all along, even while it hurt. What we are glad of is that it is past. If the relief is rational, there is a fact for it to be about, and that fact is tensed.

The future is open. Prior also studied branching models of time, in which there is one past but many possible futures, an idea he developed after a suggestion from the young Saul Kripke. On such a model "there will be a sea battle tomorrow" need not yet be either true or false.

Where he meets Deutsch

Deutsch agrees, emphatically, that the future is open. Relative to any observer, the past is fixed and possibilities really do become actualities. He says spacetime physics cannot accommodate this, and that it took the multiverse to make sense of it. Prior's branching tree and Deutsch's multiverse are reaching for the same structure.

Both also take seriously statements about what might have been. Prior's branches give "might" a home in logic. Deutsch's give it a home in physics: a claim about what would have followed had Faraday died before his great discovery refers to universes in which he did.

And Deutsch would grant the headache its point in one sense. From the point of view of an observer at a moment, that moment is singled out as "now", just as any place is "here". Relief is had by a particular snapshot of a person, about what lies before it.

Where they part

Presentism. This is the sharpest disagreement on the site so far. For Prior, only the present is real. Deutsch calls the belief that the moment of which we are aware is the only real one, or even a little more real than the others, a form of solipsism. All moments are physically real.

Which branches are real? On Prior's tree, there is one actual history and the other branches are possibilities. In Deutsch's multiverse, the branches all exist, in definite proportions. What is open for an observer is closed for none of them.

Is there a moving present? Prior's tense logic is built around a present that advances and a truth that changes. Deutsch holds that nothing advances, even subjectively. What we experience are differences between perceptions and memories.

Logic or physics. Prior begins with how we reason and speak, and defends tense as indispensable to it. Deutsch treats the tenses of ordinary language as constructs we evolved for talking about the snapshots closest to ours: useful, but no guide to what is fundamental.

See every view compared →

Further reading

• A. N. Prior, "Thank Goodness That's Over", Philosophy 34 (1959).
• A. N. Prior, Past, Present and Future (Oxford, 1967).
• Peter Øhrstrøm and Per Hasle, Temporal Logic: From Ancient Ideas to Artificial Intelligence (Dordrecht, 1995), on Prior's place in the history of tense logic.
• Arthur Prior, Stanford Encyclopedia of Philosophy.