 Ockham then radicalised these critiques by denying that there are any such species at all: perception and other phenomena which were usually explained by species – the sun heating or illuminating physical objects, for example – were now explained by action at a distance (Tachau 1988, pp. 130ff., Stump 1999). There was a similar debate about the causal mechanisms behind memory, where, again, Ockham denied a species-based account; however, in the case of memory, he replaced species not with action at a distance but with habits (Wolter and Adams 1993).