 The Epicureans also seem to have come up against another enduring problem: that of the underdetermination of theory by data. If the senses cannot directly “bear witness” to the existence of what cannot be observed, being able only to bear witness against it, there may be a number of apparently equally acceptable hypotheses about the unobservable. One could argue, as Epicureans seem to have done, that these should all be regarded as true, in the sense of being realized somewhere and at some time. But this seems unsatisfactory when it comes to explaining particular events.