Thomas Reid was a Scottish philosopher of the Enlightenment.

Like his contemporary, David Hume, he rejected the idealism of other philosophers in his time. Though they differed on the details, Reid and Hume both rejected the conclusions of Descartes and the philosophers that followed him, which described reality as consisting only of ideas in the mind. Reid, in particular, held that common sense, in the philosophical sense, is the neccessary foundation for all knowledge. Reid believed that Descartes, and those who followed his line of inquiry, were doomed to find themselves in a state of complete skepticism, where all external reality is doubted.

Reid beleived that such a philosophy of true skepticism, or a lack of belief in the external world, is an unlivable philosophy. He explains in his Inquiry:

"[...] what is the consequence? I resolve not to believe in my senses. I break my nose on a post that comes in my way; I step into a dirty kennel; and, after twenty such wise and rational actions I am taken up and clapped into a mad-house."

Common Sense

Thomas Reid founded the Scottish School of Common Sense, and promoted common sense as the true foundation of knowledge, which comes even before reason.

By common sense, Reid refers to the things that all human beings know, and must know, in order to function. He defined six axioms, or pieces of knowledge that all humans posess, that are neccessary assumptions for any reasoning to take place. To Reid, no human being can honestly deny believing in these axioms:

1. That the thoughts of which I am conscious are thoughts of a being which I call myself, my mind, my person;
2. That those things did really happen that I distinctly remember;
3. That we have some degree of power over our actions, and the determination of our will (That is, we have free will, at least to some degree);
4. That there is life and intelligence in our fellow men with whom we converse;
5. That there is a certain regard due to human testimony in matters of fact, and even to human authority in matters of opinion;
6. That, in the phenomena of nature, what is to be, will probably be like what has been in similar circumstances. (That is, that the future will resemble the past.)

Selected Works

• An Inquiry Into the Human Mind on the Principles of Common Sense (1764)
• Essays on the Intellectual Powers of Man (1785)
• Essays on the Active Powers of Man (1788)