we show that einstein's explanation of photoelectric effect is a causal explanation that has clear epistemological advantages with respect to other rival explanations. we point out the heuristic fertility of this hypothesis and the problems brought about by the acceptance of the reality of light-quanta. we conclude that the reality of light-quanta has been historically affected by the general problem of the reality of quantum entities and that the former cannot be solved independently of the latter. for that reason, the causal component of einstein's explanation cannot be retained in the context of quantum theory of particle motion. in any case, the original heuristic value of einstein's hypothesis of light-quanta is not affected by the aforementioned considerations.