I have a question about contingent sentences. To my knowledge, contingent sentences are sentences that are neither tautologies nor contradictions.
In a textbook I read that a sentence might always be true and still be contingent.
The example given was as follows: If there never were a time when the universe contained fewer than seven things, it follows that the sentence "At least seven things exist" would always be true. Yet the sentence is contingent; its truth isn't a matter of logic. There is no contradiction in considering a possible world in which there are less than seven things.
Could someone please explain why this is the case? As well as explain the example to me?