I'm reading Samir Okasha's article "Does Hume’s argument against induction rest on a quantifier-shift fallacy?" and in page 240 there is this:
Consider a typical inductive inference of the sort Hume was interested in, e.g. from ‘The sun has risen every day in the past’ to ‘The sun will rise tomorrow.’ If we add in the premise ‘Nature is uniform’ or ‘The future will resemble the past’, we may perhaps be able to make the inference deductively valid. But it is not true that this premise is needed to convert the argument into a valid one: any number of additional premises could be invented that would do the trick. No logically invalid inference has the property that there is only one additional premise which can be added to make it valid; there is always an innumerable number of such premises. If Hume meant that ‘Nature is uniform’ must be added to the premises of an argument from experience in order to make it deductively valid, as Stove and Mackie allege, it is hard to avoid the conclusion that he was guilty of an elementary logical error.
Why does he think the premise is not needed? Does he mean it is logically straightforward or obvious to convert any invalid argument into a valid one? If so, how?