Who formulated the idea of mass? 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.
 A: Mass manifests itself as 1) resistance to changes in velocity (inertial mass), and 2) the source of gravitational changes in velocity (gravitational mass). 
Galileo Galilei was the first person to formulate coherent thoughts on inertial and gravitational mass and their equivalence. His insights were based on experiments with rolling balls on inclined planes. This all happened early 17th century.
A quantitative theory for mass and motion had to await the arrival of Isaac Newton. His Principia got published at he end of the 17th century.
A: Exact history of basic science conceptions is rather obscure since it involved many cross references. 
You can try to start from this very informal and accessible article:
Historico-Critical Analysis of the Concept of Mass: From
Antiquity to Newton
It has further links to even more detailed investigations.
Google query: "the history of conception of mass"
Ps: If you are interested in ideas on which mechanics was based you can find Mach's book an invaluable source The science of mechanics; a critical and historical account of its development, by Ernst Mach. Or amazon version.
