In English, if we want to state rule or some didactic principle, we use the indefinite article. So, for example, we might say "A car drives on the right side of the road." meaning that we drive on the right side as a matter of principle or law. If we say "The car drives on the right side of the road." it means something completely different and refers to a particular situation. It's a little bit subtle. In fact, a common error of non-native speakers of English is to incorrectly use the definite article when stating a didactic principle.
How is are things phrased in Latin when expressing a principle versus a fact? Examples from classical literature would be appreciated.