But it must be understood that such practices must be justified by the circumstances.  They suppose in him who resorts thereto a right to withhold information that overrides the right of his interrogator.  If the right of the latter to know is superior, then the hiding of truth would constitute an injustice, which is sinful, and this is considered tantamount to lying.  And if the means to which we resort is not lying, as we have defined it, that is, does not show a contradiction between what we say and what we mean, then there can be no fear of evil on any side.

Now, suppose that instead of using a term whose signification is contrary to what my mind conceives, which would be falsehood, I employ a word that has a natural double meaning, one of which is conform to my mind, the other at variance.  In the first place, I do not speak against my mind; I say what I think; the word I use means what I mean.  But the other fellow! that is another matter.  He may take his choice of the two meanings.  If he guesses aright, my artifice has failed; if he is deceived, that is his loss.  I do him no injustice, for he had no right to question me.  If my answer embarrasses him, that is just what I intended, and I am guilty of no evil for that; if it deceives him, that I did not intend but willingly suffer; I am not obliged to enter into explanations when I am not even bound to answer him.  Of the deception, he alone is the cause; I am the occasion, if you will, but the circumstances of his inquisitiveness made that occasion necessary, and I am not responsible.

This artifice is called equivocation or amphibology; it consists in the use of words that have a natural double meaning; it supposes in him who resorts to it the right to conceal the truth, a right superior to that of the tormentor who questions him.  When these conditions are fulfilled, recourse to this method is perfectly legitimate, but the conditions must be fulfilled.  This is not a weapon for convenience, but for necessity.  It is easy to deceive oneself when it is painful to tell the truth.  Therefore it should be used sparingly:  it is not for every-day use, only emergencies of a serious nature can justify its employ.  Another artifice, still more delicate and dangerous, but just as legitimate when certain conditions are fulfilled, is what is known as mental restriction.  This too consists in the employ of words of double meaning; but whereas in the former case, both meanings are naturally contained in the word, here the term employed has but one natural signification, the other being furnished by circumstances.  Its legitimate use supposes that he to whom the term is directed should either in fact know the circumstances of the case that have this peculiar significance, or that he could and should know them.  If the information drawn from the answer received is insufficient, so much the better; if he is misinformed, the fault is his own, since neither genuine falsehood nor evident injustice can be attributed to the other.