St. Thomas speaks of truth as a part of justice:

Since man is a social animal, one man naturally owes another whatever is necessary for the preservation of human society. Now it would be impossible for men to live together, unless they believed one another, as declaring the truth one to another. Hence the virtue of truth does, in a manner, regard something as being due.

It is not clear whether St. Thomas intends to say precisely this, but in fact it would be impossible for men to live together without believing one another in a particular sense, namely it would be impossible for them to speak a common language, or in other words for them to communicate with one another by language at all.

Consider what would happen if people only said “this is red” about things that are blue. If this happened, “red” would simply acquire the meaning that “blue” presently has. The resulting situation would be entirely normal, except that the word “red” would have a different meaning.

Likewise, consider what would happen if people said “this is red” about random things in random situations. The phrase would cease to have any concrete meaning, and if the situations were randomized enough, the phrase would cease to have any meaning at all.

Again, supposing that one man had the intention of deceiving another as much as possible, as soon as both men are aware of this intention, the one who wishes to deceive can no longer do so. But he also cannot communicate anything; if he says, “there will be a concert tomorrow,” the other man will not believe that there will be a concert tomorrow. But neither will he conclude that there will not be a concert, because the deceiving one might have hoped for this result. Consequently he will cease to pay any attention whatsoever to what he says.

Similarly, if all men had the intention of deceiving all others as much as possible, language would simply cease to have meaning, and people would simply stop listening to one another.

Saying all of this in another way, we cannot understand the meaning of words unless they actually have some correlation with reality. This implies that it is basically necessary for truth telling to be more common than lying in order for language to exist at all; and this necessity is a necessity of fact, not merely of precept.

It follows that one harmful effect of lying is that it damages language, namely by tending to make it less meaningful. In some cases, we can see that the harm has already been done: for example, when someone asks, “How are you doing?” and the other responds, “Fine,” his response is meaningless, and it has become so on account of many past lies. And insofar as language is a common good, since it is a tool that benefits the whole community by having meaning, lying is always harmful to the common good by tending to take away meaning from the language in this way.



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