Indeed I need to reason like land surveyors do, following the principle that all other things are equal.

Frédéric Bastiat
Complete Works, Volume 7, pages 96 to 99 (in French)
Journal des Débats, May 2nd, 1846

In this short letter in response to an article published in the Journal des Débats, Frédéric Bastiat warns those who rejoice at the tariffs reductions in England that, without a similar policy in France, the benefits may well be felt by France’s competitors rather than the French people. Indeed, international trade consists in exchanging products against products, not against money (which is the objective of the mercantilists who try and favour exports while restricting imports), thus, the limitations on imports in France will increase the price of its exports, which will then be less competitive than those of other countries. He shows why with an example using mock figures.

Thus he uses the ceteris paribus clause that is absolutely necessary for sound economics (as we shall see in the sophisms Does Protection Increase the Rate of Pay? and High Prices, Low Prices). He admits that reality for sure does not reflect the situation because all other things are rarely equal. However, this does not change the fact that such a “scientific” reasoning is valid and that, if it is not possible to predict the precise economic extent of the policy (in some cases, the other things have so much more influence that the result may even seem to be contradictory to what is expected), it is possible to predict that a policy will have a “good” or “bad” effect.

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