I think the confusion here has to do with the concept of the object of a moral act.
Certainly, it is never permissible to lie. Lying is instrinsically evil as is murder. However, as noted by others here and the Catechism, there are circumstances in which is is morally permissible to deceive another. This deception is NOT lying. It is a fundamentally different object and is NOT intrinsically evil. It may appear to us to be the same object but we know in fact it is not. It shares the same seeming act in the human species but in the moral species (as defined by right reason) it is a completely different act.
Another common instance of this in moral theology is the idea of murder. The object in a murder is “the killing of an innocent human being.” The principal condition of “innocent” specifies the object to the moral order of “evil.” If we take away the condition of “innocent” and replace it with the condition of “in self defense” the object of the act is fundamentally changed. It is not properly justified murder, but rather a different act altogether that is specified to the moral order as “good” or at the very least “indifferent.” But it is not fundamentally disordered as is “murder.”
In the case lying or deceiving, we could say that the human (not moral) act is to mislead another. In the case, of lying we add the principal condition of “to lead into error.” In the case of legitimate deception, we add the principal condition of “to one who has no right to the truth.” These principal conditions specify the acts to 2 different moral categories, one good, one evil, and make them fundamentally different objects and fundamentally different acts.
By the way, this is not something I made up, this is the reasoningpulled from St. Thomas Aquinas in the Summa Theologica and Pope John II in Veritatis Splendor. Incidentally, it is this way of analyzing the structure of the moral act that is so vehemently opposed by the liberal proportionalist theologians.
I hope this was helpful and did not muddy the water too much.
To address first, we must arrive the fundamental defintion of a lie, independent of circumstance. A lie, as defined by the Catechism is: Lying consists in saying what is false with the intention of deceiving one’s neighbor (CCC2508). Our friends at Merriam-Webster take pretty much the same tack: an assertion of something known or believed by the speaker to be untrue with intent to deceive.
Once we establish that we are deceiving someone, it objectively becomes a lie. After that, St. Thomas Aquinas chimes in, and apparently, I’ve read a very different version of the book that yours.
(Housed at New Advent) Summa Theologica, Second Part of the Second Part, Question 110, Article 3: Whether every lie is a sin?
"An action that is naturally evil in respect of its genus
can by no means be good and lawful, since
in order for an action to be good it must be right in every respect: because good results from a complete cause…
Moreover, as written by HH John Paul II, in
Veritatis Splendor he wrote:
"In order to justify these positions, some authors have proposed a kind of double status of moral truth. Beyond the doctrinal and abstract level, one would have to acknowledge the priority of a certain more concrete existential consideration. The latter, by taking account of circumstances and the situation, could legitimately be the basis of certain exceptions to the general rule and thus permit one to do in practice and in good conscience what is qualified as intrinsically evil by the moral law. A separation, or even an opposition, is thus established in some cases between the teaching of the precept, which is valid in general, and the norm of the individual conscience, which would in fact make the final decision about what is good and what is evil. On this basis, an attempt is made to legitimize so-called “pastoral” solutions contrary to the teaching of the Magisterium, and to justify a “creative” hermeneutic according to which the moral conscience is in no way obliged, in every case, by a particular negative precept.
No one can fail to realize that these approaches pose a
challenge to the very identity of the moral conscience in relation to human freedom and God’s law. Only the clarification made earlier with regard to the relationship, based on truth, between freedom and law makes possible a discernment concerning this ‘creative’ understanding of conscience."
He further stated that this discernment can only come from reason borne not of interior conscience but of reason “enlightened by Divine Revelation and by faith.”