Lying is the direct and deliberate deprivation of truth from an assertion. It is the intentional choice of an act inherently directed at asserting what is false. Lying is intrinsically evil and therefore always immoral. Mental reservation, properly defined and understood, is not a type of lying.
From my book on ethics: “Mental reservation is the expression of one truth, with the reservation (i.e. the omission) of a related truth. There are two types of limitations that may cause a statement to be a type of mental reservation: (1) the expression of a truth with the omission of a related truth, or (2) the expression of a truth with the omission of the true manner of interpretation. In the first case, there are two related truths; one is expressed and another is omitted. In the second case, the related truth that is omitted is merely the proper manner of interpretation of the expressed truth. Human language often has multiple possible meanings; this commonly-understood feature of language does not cause what is expressed to be a lie.” (Conte, Catechism of Catholic Ethics, n. 213)
Mental reservation, properly understood and applied, is not intrinsically evil because no falsehood is intentionally asserted; the assertion is not inherently directed toward the deprivation of truth. Therefore the moral object of mental reservation is good, not evil.
However, in order to be moral, any act must have three good fonts: (1) intention, (2) moral object, (3) circumstances. So not every act of mental reservation is moral. If any act of mental reservation has a bad intention, it is a sin. If any act of mental reservation can be reasonably anticipated to do more harm than good in the circumstances, it is a sin.
If an act of mental reservation has a second moral object that is evil, then the act would be intrinsically evil and always immoral. For example, if you swear an oath ‘to tell the truth, the whole truth, and nothing but the truth’, the use of mental reservation would not be the sin of lying, but it would be the sin of perjury, since you swore to tell the whole truth. Some oaths include phrasings such as ‘without any mental reservation or purpose of evasion’ (e.g. the Constitutional Oath enacted in 1862 and still in use today). The use of mental reservation in such a case would be the sin of breaking an oath. If you swear that you will not use mental reservation, and you use it, you have broken your oath. The use of the mental reservation is not lying, but perhaps you lied when you said you would tell the whole truth, or would not use mental reservation (or perhaps you spoke the truth, but later changed your mind).
Lying is intrinsically evil, and therefore always wrong. This absolute prohibition against lying poses difficulties for the faithful living in the sinful world. There will inevitably be times when a faithful disciple of Christ must endure suffering because he is not willing to lie. There is no ethics without sacrifice.
Some persons hold to the erroneous doctrine that an act cannot be immoral if such a moral evaluation of the act would result in great suffering. Other persons merely lean in the direction of this error. Many persons look for a way to justify lying in order to avoid suffering. One approach is to broaden the definition of mental reservation to an excessive extent, so that, in a difficult situation, almost any assertion might be used to avoid suffering. This approach has been called ‘strict mental reservation’, but it is nothing other than a type of lying.
There are two types of strict mental reservation:
1. The direct and deliberate assertion of a false statement, qualified by the addition of words, in the mind only, that would make the statement true.
2. The direct and deliberate assertion of a false statement, qualified by the addition of an interpretation, in the mind only, that would make the statement true.
The first type of assertion is the classical type of strict mental reservation. This type of assertion is a lie because the additional words are not stated or implied. No reasonable person could hear the false statement and understand the additional unstated words needed to make the statement true. The asserted statement is false. That which is neither asserted nor implied cannot turn a false assertion into a truth.
The second type of assertion is a more recent innovation; it is sometimes called ‘wide’ mental reservation to try to avoid the (correct) conclusion that it is essentially no different from the classical form of strict mental reservation. The only difference here is that the qualification necessary to make the statement true is a special interpretation, not a word or phrase, and the interpretation is not accessible to reason from what is stated. Again, whatever is unasserted and unimplied (whether words or special interpretation) cannot turn a lie into a truth. A statement made under true mental reservation can require proper interpretation to be true, but the interpretation must be accessible to reason. For the entire moral law is open to reason, and nothing moral is beyond the reach of reason. A reasonable person must be able to understand the truth that is asserted without access to the mind of the one making the assertion. If the correct interpretation is outside of reason, then it is outside of the statement and the statement is false.
So both types of strict mental reservation explained above are really types of lying. The direct and deliberate expression of a falsehood, or the direct and deliberate denial of a truth, are not types of mental reservation, but types of lying. One cannot merely label an assertion ‘mental reservation’ or ‘wide mental reservation’ in order to justify it. Changing the name of strict mental reservation to wide mental reservation or broad mental reservation or some other term does not make it moral. (But this is such a common approach to ethics today. A sinful act is given a new label, to make it seem moral. “No, my act is not ‘bank robbery.’ I’m merely making a withdrawal from a bank where I don’t have an account.”)
No false statement can be made true solely by what is unstated. If such were the case, every false statement could be claimed to be true by the use of a special interpretation, inaccessible to the reason of the listener, reserved only in the mind of the speaker. But this type of mental reservation is nothing other than a lie. If a person directly and deliberately asserts a falsehood, then he has lied.
Sometimes moral persons must suffer for the sake of morality. A clever use of true mental reservation does not exist for each and every difficult situation. Sometimes one must either tell the truth, like Jesus before Pilate, or remain silent, like Jesus before Herod.
A gravely harmful consequence occurs when mental reservation is defined and used so broadly that the listener would have no reasonable way to perceive the true meaning: trust is lost. Would you confide in a priest if he assured you that your words would remain confidential? What if the priest holds to an excessively broad definition of mental reservation? You could never be sure that what you understood from his assertion on confidentiality is actually true. And such is the case for communication with spouses, friends, teachers, co-workers, government leaders, or any other persons. If mental reservation is so broad that the truth of a statement cannot be understood by a reasonable listener, then nothing anyone says can be trusted. The person might have reserved some special interpretation in their mind that gives the statement the opposite meaning to what was actually said. This loss of trust proves that any overly-broad definition mental reservation is contrary to justice and truth and the will of God for humanity.
When the definition of mental reservation is so broad that the listener cannot reasonably understand truth, but only falsehood, then it is not really mental reservation. The truth must be accessible, from the statement itself, by what is stated or implied or by some interpretation open to reason. Otherwise, we could never trust any statement by anyone.