disagreed with Hume, who asserted that we can never know what an external world consists of as our knowledge is limited to the ideas in the mind, and George Berkeley, who asserted that the external world is merely ideas in the mind. By contrast, Reid claimed that the foundations upon which our sensus communis are built justify our belief that there is an external world. In his day and for some years into the 19th century, he was regarded as more important than Hume. He advocated direct realism, or common sense realism, and argued strongly against the Theory of Ideas advocated by John Locke, René Descartes, and (in varying forms) nearly all Early Modern philosophers who came after them. He had a great admiration for Hume and had a mutual friend send Hume an early manuscript of Reid's Inquiry. Hume responded that the work "is wrote in a lively entertaining manner," although he found "there seems to be some Defect in Method", and he criticized Reid's doctrine for implying the presence of innate ideas. (pp. 256–257) Thomas Reid's theory of common sense Reid's theory of knowledge had a strong influence on his theory of morals. He thought epistemology was an introductory part to practical ethics: When we are confirmed in our common beliefs by philosophy, all we have to do is to act according to them, because we know what is right. His moral philosophy is reminiscent of Roman stoicism in its emphasis on the agency of the subject and self-control. He often quotes Cicero, from whom he adopted the term "sensus communis". Reid's answer to Hume's sceptical and naturalist arguments was to enumerate a set of principles of common sense (sensus communis) which constitute the foundations of rational thought. Anyone who undertakes a philosophical argument, for example, must implicitly presuppose certain beliefs like, "I am talking to a real person," and "There is an external world whose laws do not change," among many other positive, substantive claims. For Reid, the belief in the truth of these principles is not rational; rather, reason itself demands these principles as prerequisites, as does the innate "constitution" of the human mind. It is for this reason (and possibly a mocking attitude toward Hume and Berkeley) that Reid sees belief in the principles of common sense as a litmus test for sanity. For example, in The Intellectual Powers of Man he states, "For, before