78. II. Sophisms of Deduction. -- (1) Sophisms of terms. -- These are connected with the signification of words which are changed, distorted out of their true sense, or taken in several different senses. The principal ones are:

(a) Equivocation, or ambiguity of terms. This consists in employing a word in a double sense in reasoning, or taking an ill-defined word in two different acceptations. Example: The use of the terms, liberty, equality, solidarity, evolution, rationalism, liberalism, socialism, etc. -- Equivocation introduces a fourth term into the reasoning.

(b) Passing from the collective to the separate sense (fallacia compositionis). This sophism consists in affirming of things collectively what is true only of those same things taken separately. As when Christ says, in the Gospel: "The blind see; the lame walk upright; the deaf hear"; this can only be true taking these things individually, not comprehensively.

(c) Passing from the separate to the collective sense (fallacia divisionis). This consists, on the contrary, in taking the collective sense. E. g., if we should argue: Five is one number; but two and three make five; therefore two and three make one number.

(2) Sophisms of inference or of "deduction." -- (A) The petitio principii is when we begin by supposing the very thing which is in question. This sophism is committed when we take for granted: (a) the very thing that has to be established; (b) the whole, when a part of it remains to be proved; (c) a part of what has to be proved as a whole; (d) each one of the parts of the whole that has to be established; (e) a point of doctrine necessarily bound up with the principle in question.

(B) The vicious circle is an aggravation of the same sophism: it not only takes for granted what is in question, but it proves two propositions one by the other reciprocally. E. g., Descartes proves God's veracity by the evident character of truths, and the same evident character by God's veracity.

(C) The sophism of accident confounds (a) what is accidental with what is essential, or (b) what is relatively with what is absolutely true. As when a thing is condemned absolutely because of certain abuses to which it gives occasion.

(D) The sophism of non-cause confounds concomitance or succession with the relation of causality: with this, therefore, because of this; or, after this, therefore because of this -- or again, concomitance with identity: with this, therefore this itself.

(b) Connected with this sophism is the confusion of condition with cause, or of partial with total cause.

(E) The sophism of interrogation consists in joining together several questions which are really distinct, as if they all demanded a single reply. As: Why did you kill your wife? -- This interrogation supposes already settled the preliminary question: Did you kill your wife?

(F) Ignoring the state of the question, ignoratio elenchi. This sophism takes three forms: the reasoning proves too much, or too little, or it proves something apart from what is required to be proved.^{1}

{1} To this list of the commonest sophisms may be added the paradox. This is a judgment which contradicts a common opinion. The latter may be true or false. Hence, there are two classes of paradoxes, of which only the first deserves to be so called. To maintain that all intelligences are equal (Helvetius) is a genuine paradox. -- To say that it is better to suffer wrong than to do it, at first seems paradoxical, but is not really so. The paradox is sometimes a mere joke or an ill-natured sarcasm, and is thus of no particular importance.