The previous chapter concentrated on the simple situation where objects propelled in a horizontal direction continue to move in that direction along a surface. The chapter unearthed a range of variables that children regard as relevant to the speed of motion. It also identified a series of mechanisms, of varying adequacy, which children refer to. The chapter showed that understanding of both variables and mechanisms improves with age. However the improvement is not indicative of a dependent relation. On the contrary, all the evidence pointed to two encapsulated systems, developing in parallel but completely autonomous. Seeking to explain this, the chapter made reference to the primitive notions of speed that children seem to hold, notions that rely on overtaking and stopping position rather than distance per unit time. The point was made that such notions preclude a direct analysis of speed change, rendering it instead a matter of inference. It was proposed that mechanisms are seen by children as the means to such inference, and certainly there was a close tie-up between beliefs about mechanisms and judgments of change. However, if speed and speed change are separated and mechanisms are seen to bear on the latter, speed must operate as an atheoretical construct. The same must therefore apply to its determining variables.