Lying is an intrinsically evil act. Therefore, lying is always a sin. But every sin has three fonts of morality (intention, moral object, circumstances). The evil moral object makes the act intrinsically evil and always immoral, but the other fonts are not irrelevant. A lie that would otherwise be only a venial sin becomes a mortal sin, if it is said with a malicious intention, or if the circumstances include harm that gravely outweighs any good consequences.
Therefore, there are two ways to define any intrinsically evil act:
(1) based on the essential moral nature of the act itself, as determined by its moral object,
(2) based on the moral object and one or both of the other fonts.
The CCC and other magisterial sources often define an intrinsically evil act by describing not only the moral object, but also one or both of the other fonts. For example, euthanasia is defined as murder with the intention (purpose) of relieving all suffering. By definition, we call an act euthanasia only if it has both a particular evil moral object and a particular intention. But suppose that, after a sin of euthanasia has been committed, we discover that the intention was not to relieve suffering, but to gain an inheritance. Since the intention was not to relieve suffering, the act does not meet the definition of euthanasia. But the moral object is unchanged. The act is merely a different type of murder.
The example of euthanasia proves that, even when we use the second type of definition of an intrinsically evil act, the fonts of intention and circumstances do not determine the moral object at all. Nor can a change in intention or a change in circumstances change the moral object. A change in intention or circumstances might change what we call the act, but the act remains intrinsically evil and always immoral, because the moral object is unchanged.
Abortion is defined as murder in the circumstance that the innocent victim is prenatal. If the innocent victim is killed after birth, the act is now defined as infanticide. A change in the circumstances (the age of the victim) changes what we call the act, but the moral object is unchanged. Intrinsically evil acts are always immoral, regardless of intention or circumstances.
The CCC describes lying by stating its moral object, speaking a falsehood, and by stating the most common intention, to deceive (n. 2482-83). This definition is the second type, in which both the nature of the act and one (or both) of the other fonts is described. But this does not imply that lying without the intention to deceive is not intrinsically evil, or is not a sin. Veritatis Splendor teaches that intrinsically evil acts are always immoral, regardless of intention or circumstances.
We could also define lying without reference to the most common intention (to deceive), by describing the nature of the act itself, in terms of its moral object. Now just as any doctrine of the Faith can be correctly described using various wordings, so also can lying be described with various words, but revealing the same evil moral object. A simple definition would be: to assert a falsehood. A falsehood can be asserted by words or other indications (e.g. nodding your head). A falsehood can also be asserted by denying a truth.
In fact, every lie can be philosophically viewed as a denial of truth. For evil cannot exist by itself. Every evil is a deprivation of some good. Evil is a lack of ‘being’. And so a lie is essentially a deprivation of truth from an assertion. Furthermore, every evil moral object, in any intrinsically evil act, is a deprivation of some good required by the love of God and the love of neighbor as self. The moral object of lying is evil because God is Truth and because the love of neighbor is based upon the love of God.
But the second font of morality is not the moral object by itself. Moral objects do not exist apart from intentionally chosen acts. The deprivation of truth from an assertion is an intrinsically evil act and a lie, only when the act is intentionally chosen. A person lies who intentionally chooses to assert as true what he knows or believes to be false, or to assert as false what he knows or believes to be true, or even to assert that something is true without any knowledge as to whether it is true or false.
But if a person asserts a falsehood believing that it is true, he has not lied; his act is not even objectively an intrinsically evil act. For every intrinsically evil act is intentionally chosen. Note well, however, that the intention cannot change the moral object. The intended end is of the subject (the person who acts). The moral object is inherent to the chosen act itself. The intentional choice of a particular type of act is, inherently — whether you admit it or not — a choice of the concrete act AND its essential moral nature as determined by its moral object. When that moral object is evil, the essential moral nature is also evil, and the intentional choice of that act is objectively always a sin.
Also, in every intrinsically evil act, the intentionally chosen act and the moral object have a certain relationship, such that the act is inherently directed toward that moral object as a proximate end (not merely as an intended end or purpose, and not merely as a consequence or effect). In other words, the relationship between the act and its moral object is direct.
So every intrinsically evil act is the direct and voluntary deprivation of some good required by the love of God and the love of neighbor as self. Lying is always intentional; it is always a deliberate or voluntary act. In lying, the deprivation of truth is always direct. An indirect deprivation of truth is called mental reservation. The direct and voluntary deprivation of truth from an assertion is intrinsically evil and always immoral. Mental reservation is sometimes moral and other times immoral, but it is not intrinsically evil.
Every intrinsically evil act can be defined this same way. For example, Pope John Paul II defines murder in these same terms:
“the direct and voluntary killing of an innocent human being is always gravely immoral…. The deliberate decision to deprive an innocent human being of his life is always morally evil and can never be licit either as an end in itself or as a means to a good end.” (Evangelium Vitae, n. 57).
Murder is an intrinsically evil act. Murder is defined as the direct and voluntary deprivation of life from an innocent human being. The intended end never justifies murder because no intrinsically evil act is ever justified by intention. The circumstances never justify murder because no intrinsically evil act is ever justified by circumstances. Similarly, lying is the direct and voluntary deprivation of truth from an assertion. No intention and no circumstance can justify any lie, because the moral object of an intrinsically evil act is never determined or changed or justified by intention or circumstances.
St. Augustine, in ‘On Lying’, asks whether the definition of a lie includes the intention to deceive. He leaves the question not definitively answered. But he does counsel that a person should only state what he believes to be true and should not have any intention to deceive (n. 4, last sentence). Why doesn’t Augustine simply explain that a lie is still immoral, even when the intention to deceive is absent? It is because the development of doctrine, at that point in time, had not progressed so as to make clear distinctions between each of the three fonts of morality.
Also, even if St. Augustine had answered differently, saying that lying is only lying when one has the intention to deceive, we would be obligated to disagree with him. For the teaching of the Magisterium in Veritatis Splendor is clear and definitive: intrinsically evil acts are immoral regardless of intention or circumstances.
St. Aquinas improves upon the work of St. Augustine on lying:
“The desire to deceive belongs to the perfection of lying, but not to its species, as neither does any effect belong to the species of its cause.” (Summa Theologica, II-II, 110, 3, Reply to Objection 3).
This quote requires some explanation, using more modern terminology. The desire to deceive is in the font called intention. The effects are in the font called the circumstances (or consequences). Neither font is capable of changing the moral species (the inherent moral meaning or essential moral nature) of the act, which is determined by the moral object. In other words, intrinsically evil acts, such as lying, are always immoral, regardless of intention or circumstances.
Whenever an act is a lie, properly defined, that act is intrinsically evil and always immoral.