This article analyses two different actualist fallacies. Excluding the possibility of the invention of the fax or that of submarines well before the twentieth century is the first actualist fallacy that I examine. The second is: excluding the possibility of lunar journeys well before actual moon landings, H. G. Wells confronted Jules Verne over the question of whether an author should relate to real possibilities or to impossible fantasies. I examine this debate in the light of Wittgenstein's actualist "certainty" about the impossibility of a lunar journey and, in contrast, the possibility of such a journey that was surmised by Kepler, having read Lucian's fantasy of such a journey. Truthful fictions can help scientists to discover pure possibilities that predict future actual discoveries and achievements.