Proving too much

Proving too much, in philosophy, is a logical fallacy which occurs when an argument reaches the desired conclusion in such a way as to make that conclusion only a special case or corollary consequences of a larger, obviously absurd conclusion. It is a fallacy because, if the reasoning were valid, it would hold for the absurd conclusion—the judgement of fallacy is therefore largely dependent on a normative judgement of the "absurd" conclusion. A charge of "proving too much" is thus generally invoked, rightly or wrongly, against normatively-opposed conclusions, and so are often controversial at the time they are made, as in the following examples pertaining to laissez-faire economics and slavery:

• The Georgia-born American educator Henry Coppée in 1850 described in his "Elements of rhetoric" the argument that "slavery is evil because a master beats a slave to death" as proving too much; this would mean that because domestic violence against spouses and children exists, marriage or parenthood would be evil.
• Economist Walter Block states that the argument that—because perfect competition does not exist in a free market—government must intervene, proves too much: "It argues, in effect, that 1) reality does not resemble an arbitrarily contrived model of the world; 2) reality should resemble this model; and therefore 3) the government should step in, to bring the world into closer conformity with the model. But almost anything can be 'proven' with this line of reasoning. Substitute for perfect competition objects moving faster than the speed of light, or people having more than two arms and further government intervention can easily be justified. Even were it true, however, that the market is somehow deficient because the pinnacle of perfect competition has not been attained, it by no means follows that further state encroachments on the economy would improve matters."

Showing that an opponent has proved too much is a form of reductio ad absurdum.