The fallacy of petitio principii, otherwise known as ‘begging the question’, occurs whenever use is made in the argument of something which the conclusion seeks to establish. The petitio is a master of disguise and is capable of assuming many strange forms. One of its commonest appearances has it using a reworded conclusion as an argument to support that conclusion.
Justice requires higher wages because it is right that people should earn more.
(Which amounts to saying that justice requires higher wages because justice requires higher wages.)
It might seem to the novice that petitio is not a fallacy to take for a long walk; it seems too frail to go any distance. Yet a short look at the world of political discourse reveals petitios in profusion, some still running strongly after several hundred years. It is quite difficult to advance arguments for a commitment which is in essence emotional. This is why politicians deceive themselves accidentally, and others deliberately, with a plethora of petitios. The political petitio usually appears as a general assumption put forward to support a particular case, when the particular case is no more than a part of that same assumption.
The British government should prohibit the sale of the Constable painting to an American museum because it should prevent the export of all works of art.
(It looks like an argument, but the same reason could be advanced for each particular work of art. Adding them up would tell us no more than that the government should prevent the export of all works of art because it should prevent the export of all works of art.)
Argument is supposed to appeal to things which are known or accepted, in order that things which are not yet known or accepted may become so. The fallacy of the petitio principii lies in its dependence on the unestablished conclusion. Its conclusion is used, albeit often in a disguised form, in the premises which support it.
All arguments which purport to prove the unprovable should be carefully scrutinized for hidden petitios. Arguments in support of ideologies, religions or moral values all have it in common that they attempt to convince sceptics. They also have it in common that petitios proliferate in the proofs.
Everything can be defined in terms of its purpose.
(Never be surprised when a discussion starting like this ends up ‘proving’ the existence of a purposive being. If things are admitted at the outset to have a purpose, then a being whose purpose that is has already been admitted. This is a petitio prindpii disguised as a proof.)
When using the petitio yourself, you should take great care to conceal the assumption of the conclusion by skilful choice of words. Particularly useful are the words which already have a hidden assumption built into them. Words such as ‘purpose’ fall into this group. Philosophers always go into battle with a huge stockpile of these words, especially when they try to tell us how to behave. The obligations they wish to impose upon us are hidden away in words like ‘promise’. It looks like a straight, factual thing, but it has an ‘ought’ tucked away in its meaning.
The important thing to remember about the petitio is that it is supposed to look like an argument in support of a case. You should therefore spatter it with argument link words such as ‘because’ and ‘therefore’, even if it is no more than a simple rewording.
When pushed into a corner you can often effect a dramatic escape with a well-chosen petitio by combining both the assumption of a general truth, and a rewording of the conclusion.
We should not sell arms to Malaysia because it would be wrong for us to equip other nations with the means of taking human life.
(This looks and sounds like an argument, but it is really just a clever way of saying that we should not sell arms to Malaysia because we should not sell arms to anyone.)