For empiricists claiming that all knowledge of the existence of objects and their properties has to be derived from ideas of sensation and reflection it may appear as a challenge to account for potentialities and potentials. For potentialities and potentials are not perceivable. In attributing powers etc. to objects we attribute properties typically only actualized in the future. Regarding our knowledge of objects, Locke, Berkeley and Hume claim that we take a number of perceived qualities to be united in, or to constitute, one thing. Locke believed that experience permits us to infer the existence of material and spiritual substances as unknown bearers of perceived qualities, which, in their turn, count as powers—dispositional properties. Thus, on a Lockean account, experience will license the attribution of second-order dispositional properties to objects. Berkeley, however, denied the existence of material substances and claimed that sensible objects are mere bundles of qualities. But to the extent Berkeley can account for the difference between appearance and reality and the transpersonal, cross-temporal perceptibility of objects he may well be able to account for dispositional qualities as well. Hume for his part denies that anything can be known about supposedly necessary aspects of existence or occurrence. Moreover, his account of causality reveals him to be a sceptic concerning powers. Hence, within a Humean ontology, it would be difficult to make sense of dispositional qualities or potentialities.