Thomas Reid (1710-1796) was a philosopher best known as the founder of the "Scottish school of common sense," a philosophical movement very prominent in the English-speaking world and France in the late 18th century and early 19th century. He was also a leading figure of the Scottish Enlightenment, and in his day and for a few generations afterwards was internationally regarded as among the very most important philosophers. He is, along with the English philosopher G. E. Moore, probably the best-known proponent of the general view that common sense serves as a foundation for knowledge. Reid published three well-known books: the relatively brief An Inquiry into the Human Mind, which concerns mostly epistemology and philosophy of perception, the much longer Essay on the Intellectual Powers of Man, a very broad work in metaphysics and epistemology, and An Essay on the Active Powers of Man, concerning ethics and what we would now call action theory.
To understand the view that common sense serves as a foundation for knowledge, it is very important to note what "common sense" means, however, for Reid (and his followers). While Reid's definition probably will not clarify matters for the average reader, he did lay out a number of signs that a certain proposition is a "first principle" known by "common sense." Generally, they are propositions that we cannot help but believe, if we are psychologically healthy, even if out of skepticism we try. Moreover, denial of the principles of common sense is absurd and so we naturally ridicule their denial; it is possible to construct ad hominem and ad absurdum arguments for them; and they are generally agreed to around the world and throughout history. So these propositions are not the sort of homely truths and folk wisdom, such as that chicken soup is good for colds; they are much more fundamental propositions, which we are supposed to believe due to our nature as human beings, such as that there are physical objects we can bump into, and that 1+1=2. In short, they are things that nature supposedly makes us all believe.
Of course, many philosophers, perhaps most, have rejected the coherence or the philosophical usefulness of the notion of common sense, and scientists have even less respect for the view. It seems question-begging to many people, particularly when Reid (or Moore for that matter) attempts, for example, to refute skepticism simply by saying that it is a matter of common sense that an external world exists independent of our perceptions and beliefs.
However that might be, Reid has enjoyed an increasingly positive reputation in the last 40 years or so. This is probably due to the common view among analytical philosophers, popularized especially by G. E. Moore, that basic "intuitions," or opinions about specific cases, are the "data" we use to decide how to explain and argue for various accounts of concepts. While this is not at all the same as Reid's approach--Reid was perfectly willing to endorse general principles, not judgments of particular cases, as the data or foundation of his philosophical work--both views are willing to endorse the idea that the opinions we naturally find ourselves with are what we have to work with, in philosophy. Particularly in the field of epistemology, philosophers like Roderick Chisholm, William Alston, Alvin Plantinga, among many others, have responded to and developed Reidian insights.
- Thomas Reid (1801)An inquiry into the human mind: on the principles of common sense
- In Essays on the Intellectual Powers of Man, Essay VI, Ch. II: "This inward light or sense is given by Heaven to different persons in different degrees. There is a certain degree of it which is necessary to our being subjects of law and government, capable of managing our own affairs, and answerable for our conduct towards others: this is called common sense, because it is common to all men whom we can transact business with, or call to account for their conduct." Sixth Edition, ed. William Hamilton and James Walker (Phillips, Sampson, and Co.: New York, 1855), p. 352.
- Ibid., Essay VI, Ch. III. Reid proposed some other earmarks of common sense as well.
- Cf. Roderick Chisholm, "The Problem of the Criterion," in The Foundations of Knowing (Minneapolis, MN: University of Minnesota Press, 1982), pp. 61-75: using Chisholm's terminology, Reid might be described as a "methodist" while the common analytic view is "particularist." See also The Problem of the Criterion.