More thoughts on universal laws

My feeling is that if the concept of gravity becomes 'petrified' or a priori knowledge, then it must in effect become some form of 'law'. It does seem to become both nomological and axiomatic, both in the way we use the term and in its logical / rational meaning.

I am suggesting that statements such as "the law of gravity is not universally true" takes on similarities with statements such as "2+2=5". We can understand the statement but we instantly know it is false, in a scientific sense of false, just as we know 2+2=5 is false in a mathematical sense but still has a kind of meaning.

By law I mean something that is universally true, and as such any rejection of the law involves a contradiction. A universal law is a necessary rather than a contingent truth.

My understanding is that Hume wouldn't deny the existence of gravity, but he would deny that it is, in any sense, necessarily true. He would say that we appear to see a necessary connection between dropping an apple and it falling towards the Earth, but in fact this connection has been formed through habit. We have no other reason to expect that gravity will continue to apply in the future other than habit or constant conjunction of the ideas.

Having just disagreed with my favourite philosopher (Hume) I must admit that I am changing the rules slightly by which we use the term necessary. In effect I think I am trying to argue that there are either more types of knowledge than the traditional ideas of just a a posteriori and a priori, there is something in between, or empirical facts can mutate into a priori facts in the case of universal laws.
I agree that there is a lot of similarity with this and Kant's metaphysical ideas about space and time, but there are also significant differences.

The question I suppose is, what is my justification for saying that the law of gravity is a universal law that is necessarily true. There could be a number of justifications such as for example:

• the weight of evidence for past events
• the success of predictions using the law
• the lack of any reason or evidence to suppose it to be false
• a foundational belief
• an inate idea
• common sense

If we reject all of the above and stick with Hume, one further thought springs to mind. Why is Hume happy to accept that 2+2=4 will always be true if he is not happy to accept that gravity is a universal law for our universe?