Oh, a very good link. Thanks! I prefer the idea that Newton's 2. law is "half a law". It is a complete theory only when it is combined with other descriptions of force such as F=Gm1m2/r^2.
F=ma
How do we define m and F? If their definition both come from this equation, then the equation doesn't really mean anything...We can also definite a F', and say F'=ma^2, and there isn't anything wrong... However, we have F=ma as something true, then F and m must be something true...