In certain ways, Reid and Hume are closely associated as Scottish philosophers who attempted to answer important metaphysical questions primarily with recourse to reason and empirical evidence, rather than relying on theological concepts established a priori. They differ though in their accounts of epistemology and their placing of the burden of proof concerning knowledge.

For Locke and Hume, the mind operates primarily on ideas. In other words, we have physical sensations, form ideas of those senstations in our mind, and then cogitate about ideas. For Hume, all of our knowledge is grounded in sensation recollected as experience or memory. Even the abstract idea of a self is, to a degree, a product of combination of simple ideas.

Reid eliminates the intermediary of ideas in his account of knowledge. Rather than believing us to think about distinct mental objects ("ideas") which we construct from the objects of sensation, Reid considers that we think directly about sensations. On the other hand, unlike Hume, he argues for a self which contains a priori common sense by virtue of its constitution, in some ways moving towards a Kantian position that would become increasingly important in later periods of the Scottish Common Sense school.