Our current conception of gravity presupposes certain mathematical steps which it is 'logical to take'. A mathematical assumption is that zero is not sufficient on its own to explain what we want to explain; so we invent 1 and thus invent and presuppose the other side of the equation, so we can constantly relate -1,0 and 1 to other created entities. This helps us to explain 'reality' (which is, potentially, infinitely variable/infinitely inexplicable and infinitely simple at the same time - i.e. answer to the life, universe and everything = 42 - this means nothing and is unsatisfactory to many people) so we continue to go on the base assumptions of maths. They are working according to deductive logic. However, they are not justified. Gravity can be zero on its own, the only side of the equation, which ENTAILS a potentially infinite amount of point of views of it. Numbers assigned to it have, themselves, been created by a civilisation which cannot justify its own numbers being correct. So all we can assume (which we should never desire to do anyway) is that gravity may be universal, the concept, the idea. If it is, we should not even hope to define it until we have experienced every point of view (of our current reality) simultaneously....thus looking inside and outside at the same time and producing the third force.
Just a few thoughts