Realism and locality are defined emphasizing the fundamental character of both principles. From this, the well‐known expression proposed by Bell for calculating correlations is derived. Starting from this expression, the theorem of incompatibility is proved between local realism and quantum mechanics. An analysis is made of the experiments performed by Aspect and others, and it is concluded that, because photomultipliers and polarizing filters are not perfectly efficient, it is necessary to make subsidiary assumptions about how they operate in order to rule out local realism. It is argued that the subsidiary assumptions are not logically necessary and, consequently, local realism has not yet been refuted empirically.