This may sound like a dumb question, but googling didn't seem to help. Mostly because I'm having a hard time formulating the question, and anything with "mass" seems to direct me to "Mass Effect".
I am trying to find out if we know who (philosopher, scientists, or just civilization) formulated the idea that objects which are larger or heavier than others have more "stuff" in them which makes them heavier. I realize that cavemen would have known this since rock A is harder to move then rock B. But I want to know who is credited with actually formulating a theory of mass.