Please excuse this question from a lay person but if I don't ask I will never know. I know that the speed of light is a constant, that no matter how fast an object travels, light travels away from that object at the same speed. Hence nothing can travel at or faster than the speed of light, time dilution, etc., etc. What I don't understand is how Einstein, in his famous "thought experiment", could infer that the speed of light is constant and that he could "never catch up with it". How could he infer that simply from a thought experiment with no physical measurement of the speed of light available to him. Or is the thought experiment just a metaphor? Thanks.