Immanuel Kant suggested a division between “analytic” and “synthetic” truth. A classic example of the difference would be the two statements: “All bachelors are unmarried”, as against “All bachelors are sexually frustrated”. The former is necessarily true by definition; the latter may or may not be true, and needs to be justified or falsified by evidence.
All historical statements are necessarily synthetic; even such seemingly basic ones as “Edward Heath never married”, “Hitler hated Jews”, or “The Battle of Hastings took place in 1066”. This is because the truth, or otherwise, of these statements is dependent on the evidence available. Furthermore, because I cannot myself prove or disprove any of them from my own personal experience, I am entirely dependent on what other people have judged to be reliable evidence.
The “postmodern” school of historiography, which has emerged in recent decades, has argued from this that accurate historical knowledge is impossible, since even well-placed contemporary witnesses might, for all we know, be biased, lying or simply wrong. All any document tells us for sure is the state of mind of the person who wrote it. It is therefore meaningless to ask whether any fact about the past is either true or untrue; there are no facts, only opinions; all are equally valid or invalid, and if we try to judge between them all we do is reveal our own prejudices and preconceptions. Knowledge of the past is impossible, and it therefore does not matter what we say about it.
One simple argument against this thesis is its circularity. If all true knowledge is impossible, and no interpretation is necessarily valid, then this also applies to postmodernist theorising: there is literally no reason why we should consider postmodernism has any validity. However, I would prefer to use the following argument:-
I once read an official Soviet history of the 20th century, where one detail particularly struck me. It concerned the Cuban missile crisis of 1962. In the Soviet account, this became an attempt by the Americans to invade Cuba, which was thwarted by the Russians standing firm. This was not, as I first assumed, a confusion with the Bay of Pigs incident the previous year, which the book had already dealt with. I used to ask my students, why should you think that this account of the missile crisis is untrue, and the one you are familiar with is true? Can you in fact make any judgement at all?
The only valid answer I could think of is that historical writing in the West is a “free market” in ideas and interpretations, which is backed up by professional rivalry. Competition between historians means that they delight in pronouncing that their rivals have got it all wrong, so if Professor Smith produces a book putting forward controversial ideas, then Professor Robinson will take great pleasure in writing a review that says, politely or otherwise, that Smith has got it all wrong. There are fashions in historical interpretation, just as there are in clothes, and right-wing or left-wing schools of thought. Some decades ago, Jonathon Clarke proclaimed that the Whig and Marxist schools of historical interpretation were well past their sell-by date, and it was high time we had a Thatcherite school. There was never any such free market of ideas in the Soviet Union; the Communist Party alone decreed what was historically correct, and no dissenting views were permitted. Events of the past were blatantly falsified; for instance, the doctoring of old photographs to cut out awkward people like Trotsky, or later on, Stalin. Are things any better in Russian historical writing today? I have no idea.
Clive James, reviewing some years ago an official biography of the late Soviet leader Leonid Brezhnev, which contrived to avoid mentioning either Stalin or Khrushchev (and, incidentally, Khrushchev’s name never occurred in the account of the Cuban missile crisis I referred to above, nor did Kennedy's), wrote as follows, “Every euphemism, circumlocution, outright omission and flat lie is an eloquent testimonial to the Soviet government’s regard for the truth. The Soviet government has such a high regard for truth that it will go to almost any lengths to ensure that the common people never get even a smell of it”.
I wonder whether the Soviet leaders were secret postmodernists, and believed that manipulating the records was of no importance, because all interpretations of the past were equally valid? And what would a western academic postmodernist make of it? Would he say that it didn’t matter, because the Soviet depiction of events is just as valid as the western one? And, furthermore, can we prove that anything at all happened in Cuba in 1962, or even that Kennedy and Khrushchev ever existed?
In reality, of course, no-one behaves as if nothing about the past can ever be proved, and that therefore our beliefs about it are of no importance. When confronted with wildly different accounts of events given us by different newspapers, or by politicians and their spin-doctors, we do not throw up our hands in surrender and say, "We can't believe any of them, and in any case it doesn't matter, because it's impossible ever to establish the truth". Instead we use our judgement to attempt to sort out truth from falsehood, just like people have always done.