How to Win Every Argument: The Use and Abuse of Logic (2006)

124 How to Win Every Argument

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 profu-
sion, 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.