Physicall Entity or "object of study" for physics Not including candelas, there are 7 unit in Physics (6 dimensions unit, 1 dimensionless) which means Physics studying 7 basic properties as its "object of studies", that is; mass, length, space, time, electrical current, temperature, amount of matter and angle. We know each of this properties and derivative properties from it, such as speed momentum, energy and many more. But actually what we know for each of this properties (the "object of studies" for physics) is its behavior. For example, we know matter must have mass and occupying space, and mass bend space and time continuum. But do we know what exactly this mass that we studying for? Is there any serious writing that sum up in shortly what is each of this properties that we study in Physics? What I want is not writing that sum up what we know about it, but instead theory that purpose what is it actually. For example, what is mass, time, and space?