Why can’t we keep as many items as we want in working memory? It has long been debated whether this resource limitation is a bug or instead a feature. We propose that the resource limitation is a consequence of a useful feature. Specifically, we propose that flexible cognition requires time-based binding, and time-based binding necessarily limits the number of memoranda that can be stored simultaneously. Time-based binding is most naturally instantiated via neural oscillations, for which there exists ample experimental evidence. (...) We report simulations that illustrate this theory and that relate it to empirical data. We also compare the theory to several other resource theories. (shrink)