> being able to imagine something doesn't mean believing in it?

In general, no, but some assumptions come with such heavy implicit baggage that you arguably do.

An example can be, the question, "does anything matter?".

By asking that question, you have allowed for the possibility that some things matter. But if you allow for that possibility, you might as well believe it - because if it's wrong, by definition it doesn't matter that you're wrong.

This argument doesn't prove that anything matters. But it proves that you already assumed that some things matter.