In all three works, he uses precisely the same argument. The next section explains in detail why this assumption about reasoning matters. The Crucial Assumption Let us formally state the argument that Hume uses above in order to see why his assumption crucially supports his view that we never have any reason to believe any matter of fact. The alternative to any matter of fact is conceivable. If something is conceivable, then it is not a contradiction.
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.