 Bridgman found these reflections liberating as well as troubling. Basic space-time concepts are not uniquely determined a priori. For example, he noted that the notion of velocity common to classical mechanics and special relativity was not the only one in line with our intuitions of what velocity meant. Consider this alternative: “a traveler in an automobile measures his velocity by observing the clock on his instrument board and the mile stones which he passes on the road.” If we adopt such a procedure we will find that the speed of light is infinite, if special relativity is correct about time dilation: with the car going at the speed of light according to an observer standing on the road, the clock on the car will not advance at all while the car passes any number of milestones. This alternative concept of velocity would have the advantage that “there would be no limit to the velocity which can be imparted to material bodies on giving them unlimited energy”, which seems intuitively “natural and simple”. But assigning an infinite velocity to light is also “most unnatural, particularly if we favor a medium point of view.” So there was a dilemma: “all sorts of phenomena cannot at the same time be treated simply.” (Bridgman 1927, 98–100)