George Berkeley, the Irish philosopher who became a bishop, argued that land, sea, houses and our own bodies are nothing but ideas in a mind, and that no matter lies behind them. Thomas Reid once believed this firmly enough to accept Berkeley's whole system. In 1737 Reid, himself a minister's son, was made minister of New Machar, a country parish twelve miles from Aberdeen. Its people resisted him at first, because King's College, not the parish, had chosen him.
Two years later David Hume published A Treatise of Human Nature, and Reid said afterwards that until then he had never thought of doubting what philosophers taught about the understanding. What they taught was the theory of ideas: the mind never meets things themselves, only its own ideas of them, and must reason its way out to a world. René Descartes and John Locke built on that premise. Berkeley used it to remove matter, and Hume carried it on until even the mind was left as nothing but a sequence of impressions and ideas. That troubled Reid more than the loss of a material world. In 1785 he recalled the question he had asked more than forty years before: what evidence is there that all we know are ideas in our own minds? He had been looking ever since, he wrote, and had found none except the authority of philosophers.
His answer was that the reasoning from Descartes to Hume held together, but its first premise was false: we perceive things, not pictures of things. A sensation works as a sign. We are so made that the feeling of hardness in the hand makes us think of a hard body and believe it is there, with no argument in between. Reid called some beliefs of this kind the principles of common sense: that what we clearly perceive exists, that our faculties do not deceive us, that whatever begins to exist has a cause. All reasoning starts from them, so no reasoning can prove them. In An Inquiry into the Human Mind on the Principles of Common Sense (1764) he turned this on the sceptic who trusts reason and doubts the senses: both faculties came from the same maker, so there is no ground for trusting one and not the other.
The method came from natural science. The Inquiry opens by calling for an anatomy of the mind: everything known about the body came from dissection and observation, and the mind's powers must be found the same way, by watching what they do rather than by conjecture. As a regent at King's College, Aberdeen, a teacher who took one class through the whole course, Reid taught mathematics and natural philosophy as well as logic and ethics. His mother's family, the Gregorys, produced several professors of mathematics, and in 1748 he argued in the Royal Society's Philosophical Transactions that equations cannot measure virtue or pleasure. In 1758 he and his cousin John Gregory, a physician, founded the Aberdeen Philosophical Society, where Hume's arguments were debated.
In 1764, the year the Inquiry appeared, Glasgow elected him to the chair of moral philosophy Adam Smith had left. Hume, shown part of the manuscript before publication, answered politely. Joseph Priestley, the chemist and dissenting minister, attacked the book at length, and Immanuel Kant listed Reid among the critics who had missed the point of Hume's problem. In the nineteenth century his philosophy was taught in the colleges of North America and championed in France by Victor Cousin. Sir William Hamilton, the Edinburgh professor, edited his works. When John Stuart Mill attacked Hamilton in 1865, Hamilton's reputation fell quickly; Reid's had long been waning, and by the early twentieth century he was largely forgotten.
In the twentieth century G. E. Moore, the Cambridge philosopher, took up the defence of common sense against scepticism. Roderick Chisholm, an American philosopher, revived Reid's account of free will, in which the person who chooses is the cause of the choice. Alvin Plantinga, Nicholas Wolterstorff and William Alston, philosophers of religion, built on his case that some beliefs are reasonable without proof.
At about seventy Reid stopped lecturing and turned his Glasgow lectures into the two volumes of Essays, on the intellectual powers in 1785 and the active powers in 1788. In 1796 he visited friends in Edinburgh, then came home, where he worked in the garden and at problems in algebra. He died in Glasgow on 7 October 1796, at eighty-six.