The doctrine, set forth by David Hume and his successors, that percepts and concepts constitute the sole objects of knowledge, with the objects of perception and the nature of the mind itself remaining unknowable.
The philosophic theory that knowledge is limited to phenomena, either because there is no reality beyond phenomena or because such reality is unknowable.
In philosophy he began with a strong predilection for the physical side of psychology, and at an early age he came to the conclusion that all existence is sensation, and, after a lapse into noiimenalism under the influence of Fechner's Psychophysics, finally adopted a universal physical phenomenalism.
The relativism or phenomenalism which Hamilton afterwards adopted from Kant and sought to engraft upon Scottish philosophy is wholly absent from the original Scottish doctrine.
In Great Britain Mach's scepticism was welcomed by Karl Pearson to support an idealistic phenomenalism derived from Hume, and by Ward to support a noumenal idealism derived from Lotze.