What is Apriorism?

Normally we allow facts to be the test of our principles. When we see what the facts are, we can retain or modify our principles. To start out with principles from the first (o priori) and to use them as the basis for accepting or rejecting facts is to do it the wrong way round. It is to commit the fallacy of apriorism.


We don’t need to look through your telescope, Mr Galileo. We know that there cannot be more than seven heavenly bodies.

(This was a short-sighted view.)


The relationship between facts and principles is a complicated one. We need some kind of principle, otherwise nothing presents itself as a fact in the first place. The fallacy consists of giving too much primacy to principles, and in not permitting them to be modified by what we observe. It makes an unwarranted presumption in favour of a theory unsupported by the evidence, and therefore rejects evidence relevant to the case.


All doctors are in it for themselves. If yours really did give up all that time for no payment, then all I can say is that there must have been some hidden gain we don’t know about.

(In addition to the less well-hidden fallacy we do know about.)


Aphoristic reasoning is widely used by those whose beliefs have very little to do with reality anyway. The fallacy is the short brush which sweeps untidy facts under a carpet of preconception. It is a necessary household appliance for those determined to keep their mental rooms clean of the dust of the real world. Engraved on the handle, and on the mind of the user, is the legend: ‘My mind’s made up. Don’t confuse me with facts.’

Many of us might be unimpressed with a patent medicine for which the claim was made that recovery proved that it worked, and lack of recovery was proof that more of it were needed. We might point out that the facts were being used to support the medicine, whichever way they turned out. Yet every day precisely the same claim is made for overseas development aid to poorer countries. If there is development, that shows it works. If there is no development, that shows we must give more of it. Heads they win, tails logic loses.

The fallacy of apriorism can also be used to support a preconceived judgement against the evidence. If a politician we support is caught cheating in examinations, or in a compromising position with an intern, these are character-improving situations. They steel him and test him, making him a fitter candidate for office. For anyone else, of course, they would disqualify them from office.


Since there are no cats in Tibet, this animal here, with the ears of a cat, the tail of a cat, the fur of a cat and the whiskers of a cat, shows that Tibetan dogs are pretty good actors.

(Not only that, they also catch mice and drink milk from a saucer.)


It is generally unproductive, when using apriorism, to dismiss the facts out of hand as untrue. After all, your audience might have been there to witness them. You will go much further by reinterpreting those facts, showing how they were not what they seemed. Far from contradicting your contention, they really support it.


I still maintain that the books I recommended were the most popular ones. Of course I don’t deny that they were the least read ones in the entire library; but I take that as a sign of their popularity. You see, when a book is really popular, people buy it or borrow it from friends; they don’t wait to borrow it from a library.

(At least the fallacy is popular.)


The above article is from the book How To Win Every Argument by Madsen Pirie. The article is only for educational and informative purposes to explain and understand formal logic and logical fallacies. It is a great book, definitely worth a read!