I have been learning Newton's laws, and I am confused about the different formulations of the laws, the definitions of the quantities in the modern version, and the definitions of the quantities Newton used.
Nowadays, mass is defined to be an object's resistance to change in acceleration. But that presupposes Newton's second law.
I previously assumed that forces are and were a push or pull, and at Newton's times could be measured and represented as vectors; accelerations are and were second derivatives of displacement vectors. And Newton, with the exact previous definitions, found a relationship between the vector sum of the force vectors on a body and the acceleration vector of the body, and the constant for scalar multiplication is given the name mass, a measure of a body's resistance to change in acceleration.
However, it seems that was not the case, and mass was a pre-existing concept. But how could mass have existed when there was no Newton's second law?
Can someone give a brief explanation of the definitions of the quantities in Newton's laws? And, if the definitions changed overtime, can someone explain that change?