Optimality theory, particularly optimal foraging theory (OFT), has spurned controversy over decades. I argue that the controversy results from conceptual pitfalls. The focus in this article is on pitfalls underlying the concept of constraint. Constraints in OFT models are a means to distinguish between possible and impossible behaviours. I argue that the seemingly innocuous notion of (im)possibility is tricky. It is indeed linked here with troublesome philosophical problems concerning free will. To steer away from such problems in OFT, we need to distinguish between hard and soft constraints. Such a distinction is necessarily context-dependent. This implies that OFT, to a large extent, should take the form of natural history rather than general theory.