Sec. 7.  The Fallacy of surreptitious conclusion (ignoratio elenchi), the mistaking or obscuring of the proposition really at issue, whilst proving something else instead.  This may be done by substituting a particular proposition for an universal, or an universal for a particular.  Thus, he who attacks the practice of giving in charity must not be content to show that it has, in this or that case, degraded the recipient; who may have been exceptionally weak.  Or, again, to dissuade another from giving alms in a particular case, it is not enough to show that the general tendency of almsgiving is injurious; for, by taking pains in a particular case, the general tendency may often be counteracted.

Sometimes an argument establishing a wholly irrelevant conclusion is substituted for an argumentum ad rem.  Macaulay complains of those apologists for Charles I. who try to defend him as a king, by urging that he was a good judge of paintings and indulgent to his wife.

To this class of Fallacies belongs the argumentum ad hominem, which consists in showing not that a certain proposition is true, but that Critias ought to accept it in consistency with his other opinions.  Thus:  ’In every parish the cost of education ought to be paid out of the rates:  you, at least, have said that there can be no sound economy, unless local expenses are defrayed from local funds.’  But whether this is a fallacy depends, as Whately observes, upon whether it is urged as actually proving the point at issue, or merely as convicting the opponent of inconsistency.  In the latter case, the argument is quite fair:  whatever such a conclusion may be worth.

Similarly with the argumentum ad populum:  ’this measure is favourable to such or such a class; let them vote for it.’  An appeal to private greed, however base, is not fallacious, as long as the interest of the class is not fraudulently substituted for the good of the nation.  And much the same may be said for the argumentum ad verecundiam.  When a question of morals is debated as a question of honour among thieves, there is no fallacy, if the moral issue is frankly repudiated.  The argument from authority is often brought under this head:  ’such is the opinion of Aristotle.’  Although this does not establish the truth of any proposition, it may be fairly urged as a reason for not hastily adopting a contrary conclusion:  that is, if the subject under discussion be one as to which Aristotle (or whoever the authority may be) had materials for forming a judgment.

A negative use of this fallacy is very common.  Some general doctrine, such as Positivism, Transcendentalism, Utilitarianism, or Darwinism, is held in common by a group of men; who, however, all judge independently, and therefore are likely to differ in details.  An opponent exhibits their differences of opinion, and thereupon pretends to have refuted the theory they agree in supporting.  This is an argumentum ad scholam, and pushes too far the demand for consistency.  In fact it recoils upon the sophist; for there is no sense in quoting men against one another, unless both (or all) are acknowledged to speak with the authority of learning and judgment, and therefore the general doctrine which they hold in common is the more confirmed.