The article on Catholic Culture about the history of the most recent Catechism tells us about an interesting alternative definition of lying that was apparently considered and then abandoned. The first definition below is the one that actually appears in 2483 in the Catechism and it maximalizes the definition. The second is the one that briefly appeared in a first edition and then was dropped.
- To lie is to speak or act against the truth in order to lead someone into error.
- To lie is to speak or act against the truth in order to lead into error someone who has the right to know the truth.
Mirus does not speculate about why the drafters of the Catechism held back from this and neither will I but I think there is something worth noting.
If we look at the commandment against killing the word used in the original Hebrew means something stronger than murder but less than our English verb "to kill". The specifications the catechism makes about what counts as killing are actually a matter of bringing the church's definition closer into line with what the Decalogue says. For example, capital punishment is lawful in many jurisdictions and is not a matter of taking innocent life. That doesn't mean it's a slam dunk to say capital punishment is okay, and there are all sorts of other Catholic arguments that speak against it but it is not in violation of the decalogue. Abortion, on the other hand, is also lawful in many jurisdictions but it is the taking of innocent life so it is in violation of the decalogue.
Something very different is happening with lying. There is quite simply no commandment forbidding lying. The Decalogue forbids bearing false witness, which has a legal connotation. Our Catechism definition of lying considerably expands on that and expands it to the maximum possible connotation. Read literally, it says all untruths spoken to deceive are sins. It, as I say, maximalizes the definition.
Imagine you have a friend over and she drops one of your favourite figurines that you inherited from a much-loved relative. She feels really bad about it but you think, "Nothing is going to bring the thing back, even if she finds the very same figurine on E-Bay it won't be the one Aunt Anne left me so I will tell her a fib so that she won't feel bad". On the catechism definition that puts you in league with Satan the father of lies. (And note, this is not some extreme case that never arises in real life.)
It is worth noting that the Catholic church does the same with adultery. The definition of adultery as used in the Bible is to have sex with one person while married to another or to have sex with someone you know to be married to someone else. According to the Catholic church it is adultery to have sex with someone when you are not married because that will be adultery against the person you might eventually marry (even if you never actually marry at all).
It's easy to understand how the church gets into this position. It's a legalistic attitude that attempts to define morality entirely in terms of rules. This, combined with a bureaucratic command and control mentality, leads to the creation of rules meant to leave as little decision making as possibly up to individual Catholics. It is that maximalizing instinct that, in my opinion, causes the catechetical failure that Pope John Paul II noted had happened with Humanae Vitae and with many other aspects of Catholic moral teaching.
A final thought, there is a tradition of moral thought in the Catholic church that would help overcome these problems and it is called casuistry. Casuistry is a dirty word these days thanks to unfair attacks by critics such as Pascal but it's actually a quite sensible approach to moral catechism. It is the only approach that will work in my opinion.