I was looking through Zee's 'Quantum Field theory in a Nutshell" and he says that [itex] c [/itex] and [itex] \hbar [/itex] are "not so much fundamental constants as conversion factors." I've heard other physicists say this as well. I understand that these constants are used in some equations to give units of energy so in this sense I understand how they are "conversion factors" but we can't we still regard them as fundamental constants? Zee didn't say they absolutely aren't, but I'm not entirely sure what he's trying to get at with his distinction. That is, why is putting more emphasis on them as conversion factors than fundamental constants?