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Mental reservation:  The name applied to a doctrine which has grown out of the common Catholic teaching about lying and which is its complement.  Another good page is found at http://en.wikipedia.org/wiki/Doctrine_of_mental_reservation

Article from the Catholic encyclopedia:  

According to the common Catholic teaching it is never allowable to tell a lie, not even to save human life. A lie is something intrinsically evil, and as evil may not be done that good may come of it, we are never allowed to tell a lie. However, we are also under an obligation to keep secrets faithfully, and sometimes the easiest way of fulfilling that duty is to say what is false, or to tell a lie. Writers of allcreeds and of none, both ancient and modern, have frankly accepted this position. They admit the doctrine of the lie of necessity, and maintain that when there is a conflict between justice and veracity it is justice that should prevail. The common Catholic teaching has formulated the theory of mental reservation as a means by which the claims of both justice and veracity can be satisfied.

The doctrine was broached tentatively and with great diffidence by St. Raymund of Pennafort, the first writer on casuistry. In his "Summa" (1235) St. Raymund quotes the saying of St. Augustine that a man must not slay his own soul by lying in order to preserve the life of another, and that it would be a most perilous doctrine to admit that we may do a less evil to prevent another doing a greater. And most doctors teach this, he says, though he allows that others teach that a lie should be told when a man's life is at stake. Then he adds:

I believe, as at present advised, that when one is asked by murderers bent on taking the life of someone hiding in the house whether he is in, no answer should be given; and if this betrays him, his death will be imputable to the murderers, not to the other's silence. Or he may use an equivocal expression, and say 'He is not at home,' or something like that. And this can be defended by a great number of instances found in the Old Testament. Or he may say simply that he is not there, and if his conscience tells him that he ought to say that, then he will not speak against his conscience, nor will he sin. Nor is St. Augustine really opposed to any of these methods.

Such expressions as "He is not at home" were called equivocations, or amphibologies, and when there was good reason for using them their lawfulness was admitted by all. If the person inquired for was really at home, but did not wish to see the visitor, the meaning of the phrase "He is not at home" was restricted by the mind of the speaker to this sense, "He is not at home for you, or to see you." Hence equivocations and amphibologies came to be called mental restrictions or reservations. It was commonly admitted that an equivocal expression need not necessarily be used when the words of the speaker receive a special meaning from the circumstances in which he is placed, or from the position which he holds. Thus, if a confessor is asked about sins made known to him in confession, he should answer "I do not know," and such words as those when used by a priest mean "I do not know apart fromconfession," or "I do not know as man," or "I have no knowledge of the matter which I can communicate."

All Catholic writers were, and are, agreed that when there is good reason, such expressions as the above may be made use of, and that they are not lies. Those who hear them may understand them in a sense which is not true, but their self-deception may be permitted by the speaker for a good reason. If there is no good reason to the contrary, veracity requires all to speak frankly and openly in such a way as to be understood by those who are addressed. A sin is committed if mental reservations are used without just cause, or in cases when the questioner has a right to the naked truth.

Titius, who privately said to a woman 'I take thee for my wife' without the intention of marrying her, answered the judge who asked him whether he had said those words that he did not say them, understanding mentally that he did not say them with the intention of marrying the woman.

Navarrus was asked whether Titius told a lie, whether he had committed perjury, or whether he committed any sin at all. He drew up an elaborate opinion on the case and dedicated it to the reigning pontiff, Gregory XII. Navarrus maintained that Titius neither lied, nor committed perjury, nor any sin whatever, on the supposition that he had a good reason for answering as he did.

This theory became known as the doctrine of strict mental reservation, to distinguish it from wide mental reservation with which we have thus far been occupied. In the strict mental reservation the speaker mentally adds some qualification to the words which he utters, and the words together with the mental qualification make a true assertion in accordance with fact. On the other hand, in a wide mental reservation, the qualification comes from the ambiguity of the words themselves, or from the circumstances of time, place, or person in which they are uttered.