 In the 17th century, only Huygens came close to expressing such a view; he held that not velocity, but velocity-difference, was the fundamental dynamical quantity. He therefore understood, for example, that the “absoluteness” of rotation had nothing to do with velocity relative to absolute space. Instead, it arose from the difference of velocity among different parts of a rotating body. If a disk is translated through space without rotation, then its parts move in parallel lines, but if it is rotating, then they move in different directions, even though they are at rest relative to one another on account of the bonds holding them together. The differences, evidently, would be the same irrespective of the velocity of the body as a whole in absolute space. Unfortunately, Huygens expressed this view only in manuscripts that remained unpublished for two centuries. (Cf. Stein, 1977, pp. 9–10 and Appendix III.) Huygens also reflected on the possibility of replacing absolute space with (what we would call) empirical frames of reference, again in unpublished notes that have only been brought to light in recent work by Stan (2016). But the complete concept of the inertial frame emerged only in the late 19th century, when it did not seem to be of any great immediate importance (see below). Indeed, even after the concept of inertial frame had been widely discussed, the notion persisted that true rotation could only be understood as rotation with respect to absolute space. Poincaré, for example, convinced of the essential “relativity of space” as well as the relativity of motion, considered the concept of absolute space to be something of a philosophical embarrassment. But it was not clear to him how the dynamical phenomena of rotation could be understood without it (cf. DiSalle 2014). So the failure of Newton and Huygens to formulate the concept of inertial frame, two centuries earlier, seems less remarkable than the progress that each of them made in understanding the relativity of motion. As we will see, articulating this concept involved synthesizing (in effect) insights of Newton, Huygens, and Euler.