Thomas Reid

"For I conceive the sceptical writers to be a set of men, whose business it is to pick holes in the fabric of knowledge wherever it is weak and faulty; and when these places are properly repaired, the whole building becomes more firm and solid than it was formerly. "For my own satisfaction, I entered into a serious examination of the principles upon which this sceptical system is built; and was not a little surprised to find, that, it leans with its whole weight upon a hypothesis, which is ancient indeed, and hath been very generally received by philosophers, but of which I could find no solid proof. The hypothesis I mean, is, That nothing is perceived but what is in the mind which perceives it: That we do not really perceive things that are external, but only certain images and pictures of them imprinted upon the mind, which are called impressions and ideas. If this be true ; supposing certain impressions and ideas to exist in my mind, I cannot, from their existence, infer the existence of any thing else: my impressions and ideas are the only existences of which I can have any knowledge or conception ; and they are such fleeting and transitory beings, that they can have no existence at all, any longer than I am conscious of them. So that, upon this hypothesis, the whole universe about me, bodies and spirits, sun, moon, stars, end earth, friends and relations, all things without exception, which I imagined to have a permanent existence, whether I thought of them or not, vanish at once ; And, like the baseless fabric of a vision, Leave not a track behind. I Thought it unreasonable, my Lord, upon the authority of philosophers, to admit a hypothesis, which, in my opinion, overturns all philosophy, all religion and virtue, and all common sense ..." (From the preface to An inquiry into the human mind: on the principles of common sense)

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.

Common Sense

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.