A grand theory of motivation: Why not?

Baumeister asks what a grand theory of motivation might look like, and he identifies the key problems, challenges, and opportunities that need to be considered in its pursuit. I address four of these challenges—how to define motivation, whether motivation is a state or a trait, the primacy of motivation in psychology, and the necessity to not only manage motivational conflict but also to vitalize motivational assets. I focus primarily, however, on the key obstacle that prevents a grand theory—our non-shared assumptions about the nature and dynamics of motivation. I suggest we capitalize on new advances in statistics, methodology, and technology to test what used to be untestable assumptions about motivation. Shared assumptions are necessary for a coherent science, and only a coherent science is capable of constructing a general theory.