A wide mental reservation is using equivocations and amphibologies to imply an untruth that is not actually stated, while in the strict mental reservation (stricte mentalis), 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. The latter was condemned by Pope Innocent XI in 1679. Equivocation, on the other hand, remained orthodox, and was revived in particular by Alphonsus Liguori.
It is argued for in ethics as a way to fulfill both the obligations of telling the truth and of, in justice, keeping secrets from those not entitled to know the truth (for example because of the seal of the confessional or other clauses of confidentiality) but is regarded as unjustifiable without grave reason for withholding the truth.
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. Such mental reservation was justified by the Jesuit casuist Thomas Sanchez. It was famously denounced by Blaise Pascal in his Provincial Letters, although Sanchez added various restrictions (it should not be used in ordinary circumstances, when one is interrogated by competent magistrates, when a creed is requested, even for heretics, etc.). An anecdote often related, for instance by the canonist Martin Azpilcueta (aka Navarra) to illustrates his doctrine of a mixed speech (oratorio mixta), combining verbal speech and gestual communication, concerns Francis of Assisi who allegedly declared to people pursuing a thief that he had just seen, "he hasn't passed by here", sliding at the same time his finger in his sleeve . This type of untruth was formally condemned by Kant in On a supposed 'right to lie' .
The doctrine of mentalis restrictio or mental reservation was most fully enunciated by the 16th-century Spanish theologian Martin de Azpilcueta (aka Dr. Navarrus). Navarrus held that mental reservation involved truths "expressed partly in speech and partly in the mind," relying upon the idea that God hears what is in one's mind while human beings hear only what one speaks. Therefore the Christian's moral duty was to tell the truth to God. Reserving some of that truth from the ears of human hearers was moral if it served a greater good. The user of the doctrine could reply "I know not" aloud to a human interlocutor, and "to tell you" silently to God, and still be telling the truth (stricte mentalis).
An anecdote often related, for instance by Martin Azpilcueta, to illustrates his doctrine of a mixed speech (oratio mixta), combining verbal speech and gestual communication, concerns Francis of Assisi who allegedly declared to people pursuing a thief that he had just seen, "he hasn't passed by here", sliding at the same time his finger in his sleeve .
The doctrine of mental reservation was intimately linked with the concept of equivocation, which allowed the speaker to employ double meanings of words to tell the literal truth while concealing a deeper meaning. Navarrus did not by any means originate these ideas, but he gave them a far more broad and liberal interpretation than had anyone up to that time. Other Catholic theological thinkers and writers took up the argument in favor of equivocation and mental reservation. Though the concepts remained controversial within the Roman Catholic Church (which never officially endorsed or upheld the doctrines), the Jesuits came to favor these tactics for their obvious advantages.
The linked doctrines of mental reservation and equivocation became notorious in England during the Elizabethan era and the Jacobean era, when Jesuit agents penetrating England to maintain the Catholic cause were captured by the authorities, and used these concepts in their legal defenses. Robert Southwell (ca. 1561–1595), a Jesuit priest and agent (also a poet of note) who was arrested in England in 1592, defended the doctrines at his trial, to the predictable resistance of the authorities. (Southwell was convicted, and executed in 1595.) More famous in his own era was Henry Garnet (1555–1606), who wrote a defense of Southwell in 1598; Garnet was captured by the authorities in 1606 due to his involvement in the Gunpowder Plot. He used the same doctrines in his own defense, with the same result as Southwell: Garnet was executed that year.
The Protestants considered these doctrines as mere justifications for lies. Catholic ethicists also voiced objections: the Jansenist "Blaise Pascal...attacked the Jesuits in the seventeenth century for what he saw as their moral laxity. "By 1679, the doctrine of mental reservation had become such a scandal that Pope Innocent XI officially condemned it. Other casuists justifying mental reservation included Thomas Sanchez, who was criticized by Pascal in his Provincial Letters — although Sanchez added various restrictions (it should not be used in ordinary circumnstances, when one is interrogated by competent magistrates, when a creed is requested, even for heretics, etc.), which were ignored by Pascal. Of the 26 theses condemned by Pope Innocent XI, several were in Sanchez's works (see op. mor. in præc. Decalogi, III, vi, n. 15). One of them stated:
If anyone, by himself, or before others, whether under examination or of his own accord, whether for amusement or for any other purpose, should swear that he has not done something which he has really done, having in mind something else which he has not done, or some way of doing it other than the way he employed, or anything else that is true: he does not lie nor perjure himself.
Faith, here's an equivocator, that could swear in both the scales against either scale; who committed treason enough for God's sake, yet could not equivocate to heaven. (Macbeth, Act 2, Scene 3)
Following Innocent XI's condemnation of strict mental reservation, equivocation was still considered orthodox. The Jesuit Gabriel Daniel wrote in 1694 a reply to Pascal's Provincial Letters, titled Entretiens de Cleanthe et d'Eudoxe sur les lettres provinciales, in which he accused Pascal of lying, or even of having himself used mental reservation, by not precising all the restrictions given by Sanchez to the use of this form of deception.
This type of untruth was formally condemned by Kant in On a supposed 'right to lie’, who was debating against Benjamin Constant. The latter claimed, from a consequentialist stance opposed to Kant's categorical imperative, that:
To tell the truth is thus a duty; but it is only in respect to one who has a right to the truth. But no one has a right to a truth which injures others
On the other hand, Kant asserted, in the Groundwork of the Metaphysic of Morals, that lying, or deception of any kind, would be forbidden under any interpretation and in any circumstance. In Groundwork, Kant gives the example of a person who seeks to borrow money without intending to pay it back. The maxim of this action, says Kant, results in a contradiction in conceivability (and thus contradicts perfect duty) because it would logically contradict the reliability of language. If it is universally acceptable to lie, then no one would believe anyone and all truths would be assumed to be lies (this last clause was accepted by casuists, hence the reasons for restrictions given to the cases where deception was authorized ). The right to deceive could also not be claimed because it would deny the status of the person deceived as an end in himself. And the theft would be incompatible with a possible kingdom of ends. Therefore, Kant denied the right to lie or deceive for any reason, regardless of context or anticipated consequences.
The doctrines have also been criticized by Sissela Bok (1978) or Paul Ekman, who defines lies by omissions as the main form of lying — though larger and more complex moral and ethical issues of lying and truth-telling extend far beyond these specific doctrines. Ekman, however, does not consider cases of deception where "it is improper to question" the truth as real form of deceptions — this sort of case, where communication of truth is not to be expected, and thus justified deception, was included by casuists (for instance, in the case where deception is justified because someone would reveal one's secret crime, which, according to the morality of the times, would be "scandalous", as such a revelation would be a public incitation to sin .)
Social psychologists Janet Beavin Bavelas, Alex Black and Nicole Chovil have advanced cases, in Equivocal Communications (Sage Publications, 1990) where the actor is confronted to an avoidance-avoidance conflict, in which he both doesn't want to say the truth and doesn't either want to make an outright lie (for instance, if your friends ask you about how lovely their baby is, who is in fact very ugly..., or, in politics, case of plausible deniability), where equivocal statements are standardly preferred by locutors. They define equivocation as “...nonstraightforward communication ........ ambiguous, contradictory, tangential, obscure or even evasive” (op.cit., p. 28), and allege that people typically equivocate when posed a question to which all of the possible replies have potentially negative consequences, but where nevertheless a reply is still expected (the situational theory of communicative conflict) .