Why did Einstein choose to define speed of light as a constant and time as a "variable" (relative to motion), rather than the reverse? I am aware of experimental results confirming constant speed of light and time dilation. But Einstein did not have those results at the time he proposed SR as a theory. (I doubt he even had Morrison-Morley results, but I may be wrong on this point.) What was (or would be) so different and so vastly more difficult in theoretical physics if one were to keep time constant (relative to motion) and treat the speed of light as any other speed in physics? I guess my question is: what is a good source to look at to understand the inherent theoretical constraints that "forced" Einstein to re-define the concepts of time and lightspeed in radical contrast to Newtonian physics (including printed and Internet sources)?