Here's another interesting point, Geezer. Say we decide to use Newton's laws in an accelerating reference frame. For the sake of making your grumpy, let's assume it's a rotating reference frame. Suddenly we find that everything is experiencing a centrifugal force that is proportional to mass.

Now let's say we spent our entire lives in this rotating reference frame. We might not even realize it's rotating and we might assume that centrifugal force is some fundamental force in the universe. Eventually someone bright would come along and figure out that the magic centrifugal force can be explained by the fact that all our experiments and observations have been done in a rotating reference frame, and that centrifugal force is just an artifact of us having formulated all our theories in rotating coordinates.

Now imagine instead we grew up in a reference frame that was accelerating with respect to the natural, inertial, free-falling frame. We didn't realize we were in an accelerating reference frame, so when we formulated all our physical laws, we got this fictitious force that was always proportional to mass, and called it gravity. Then one day, Einstein came along and figured out that the "force" of gravity appears because we're working in reference frames that can be thought of as accelerating with respect to the inertial one in which gravity disappears as a force.

The analogy is quite good, actually. Fictitious forces are proportional to mass, and pop up whenever you try to formulate Newton's laws in an accelerating reference frame. Einstein's genius was to realize that gravity is also a fictitious force if you regard free-fall as the appropriate "non-accelerating" reference frame.