Thomas Reid was the seventeenth century Scottish philosopher who, during his professorship at King's College in Aberdeen, led the attack upon Humean skepticism, then both wide-based and historically deep-rooted in Scottish thought. The responsibility for the success of this attack belongs not only to Reid himself but to them extremely promising minds—whom he affected so profoundly, among them Dugald Stewart, James Beattie, James Gregory, George Campbell, and Alexander Gerard. The school lost favor under the weighty dominance of John Stuart Mill and the British idealists, and Reid's writings, as a result, have suffered from long neglect.
As is the case with the Reidian ethic, the Reidian epistemology takes its direction largely from the conflict with Humean thought. Disturbed by the prevalence of analogical hypotheses drawn from the physical world, Reid based his own epistemological theory upon his introspective analysis of the operations of the mind, outlining three possible elements in any mental act: the act itself; the act's object; an intuitive belief in the existence of the object, distinct from the mental act. On the basis of this analysis, he differentiates perception, sensation, memory, conception, and abstraction. Reid's Essays on the Intellectual Powers of Man also takes particular issue with the Cartesian and Lockean philosophy which held that the direct objects of mental acts were ideas in the mind, a point of view which necessitated the consequent attempts to provide philosophical proof to justify one's belief in physical objects, the past, other minds, etc. Reid saw in this position the roots of Humean skepticism and resisted it wholeheartedly, insisting that we have a justified common-sense belief that mental acts do have as their objects distinct physical objects, and he challenged, one by one, the skeptics' arguments to the contrary.