Being Discreet Can Be Difficult
By Father Kenneth Wasilewski
In section 2489 of the Catechism of the Catholic Church, we’re reminded that there are times when we have an obligation to guard the truth rather than share it. This might mean remaining silent or making use of “discreet language.” 
Discreet language can take different forms. It might mean speaking in generalities or steering conversations away from sensitive topics. It may mean that we reveal only a portion of what we know. 
Using discreet language may even mean employing some form of what is known as “mental reservation.” This is ultimately a form of equivocation, where my words may be at least partially deceptive, but not untrue. Thus I might be able to avoid lying in the technical sense, while still safeguarding information I have a responsibility to protect. 
That being said, throughout history Catholic theologians have disagreed upon the degree to which it can be used or even if it should be used at all. The concept of mental reservation was an attempt to find middle ground between an insistence that one should never use deceptive language and the claim that sometimes it was ok to lie if some good was being protected in the process. 
The classic dilemma that largely spawned these two lines of thought was a scenario where someone was hiding an innocent person in their home who was being pursued by those who wished to do him harm (such as those who hid Jews from the Nazi’s in World War II). If the homeowner was questioned about the innocent person’s whereabouts, what would a morally acceptable response be? 
Some theologians have claimed the response would need to be silence or an outright refusal to give information. But this position was difficult to defend if harm would still come to the homeowner or innocent person as a result. 
On the other hand, some theologians would claim it was acceptable to lie in a scenario like this. But this wasn’t an entirely satisfactory answer either since it would seemingly condone lying and in the process repudiate St. Paul’s instruction not to do evil so that good comes from it. 
The concept of mental reservation was an attempt to protect both goods at stake in scenarios like this: protecting the innocent and avoiding lying. For example, the homeowner might respond by saying something like, “I haven’t seen him in some time” (meaning, I haven’t seen him since I answered the door), or “I don’t know where he is” (meaning, I don’t know his exact location in the house right now — even if I knew what room he’s in, I may not know his exact position). Both of these statements can be considered technically true, while at the same time being deceptive — thus avoiding what technically constitutes a lie. 
The homeowner is employing a form of mental reservation by saying things that are true while reserving the full meaning of what she says to herself. Even though this idea of mental reservation isn’t included in the catechism as an actual teaching of the Church (and it isn’t an “official” teaching), it has been suggested by saints and widely taught to Catholics for generations as being an acceptable approach to avoid the extremes in the other two positions. 
Most theologians would at least agree that there is no specific Church teaching against using mental reservation, even if the Church has yet to formally endorse it. That being said, most theologians would also agree that it has its limits as well. For instance, that it should only be used for those situations that call for it — not just in a casual sense or without a good reason or for selfish gain. Likewise, it would be wrong to use it in situations where someone has a legitimate right to know the truth. 
Can a Catholic use it in good conscience in certain situations? Most theologians would say it’s an example of “discreet language,” and therefore, “yes.”