@dalehileman,
Quote:for instance conforms with “a lie” but doesn’t explain why we say “the truth"
We often, but not always, say "the truth" because we want it to be specific to either 'a truth' or a collection of truths that we are then discussing.
In such a situation, if a person used 'a' truth, it would hold a meaning/ a nuance of a universal truth. When we want to describe 'a' universal truth, there's nothing to stop us from using that meaning. This isn't so much an issue of grammar as it is a pragmatic/semantic issue.