Reason does nothing except locate the presence or absence of contradictions. We can indeed conceive of alternatives to any matter of fact. And it seems like a basic feature of a contradiction that it is inconceivable. Now try to conceive that gremlins exist. I shall briefly but formally state Hume's argument against induction, then see how it relates to his central assumption about reasoning.
All knowledge comes either from observation or reason.
Knowledge of unobserved matters of fact can' t come from reason, because the alternative to any matter of fact is conceivable and therefore implies no contradiction. Therefore, we never have any reason to believe any unobserved matter of fact. Because the contrary to every matter of fact is conceivable, conceivable things are not contradictions, and reason does nothing except locate the presence or absence of contradictions. So Hume's basic assumption about reasoning is absolutely crucial at both steps. Suppose someone had a different theory of reasoning.