 Another interesting line of research uses empirical work in psychology to better understand what is happening cognitively when we judge time as passing. This is especially pressing for B-theorists, who must explain why time seems to pass in psychologically or rationally significant ways, even though all passage is really just variation in an eternal manifold. Some B-theorists explain the apparent passage of time as an illusion of flow caused by perceptual processes that attribute apparent motion to events that happen in sequence. Another, compatible approach considers the way that evolutionary pressures might have shaped emotions and cognitive heuristics to give us a strong past/future distinction in our reasoning.