Thomas Brown (9 January 1778 – 2 April 1820) was a Scottish metaphysician.
He was born at Kirkmabreck, Kirkcudbright, where his father Rev. Samuel Brown was parish clergyman. He was a wide reader and an eager student. Educated at several schools in London, he went to the University of Edinburgh in 1792, where he attended Dugald Stewart's moral philosophy class, but does not appear to have completed his course. After studying law for a time he took up medicine; his graduation thesis De Somno was well received. But his strength lay in metaphysical analysis, as was shown in his answer to the objections raised against the appointment of Sir John Leslie to the mathematical professorship (1805). Leslie, a follower of David Hume, was attacked by the clerical party as a sceptic and an infidel, and Brown took the opportunity to defend Hume's doctrine of causality as in no way inimical to religion. His defence, at first only a pamphlet, became in its third edition a lengthy treatise entitled Inquiry into the Relation of Cause and Effect, and is a fine specimen of Brown's analytical faculty.
In 1806 Brown became a medical practitioner in partnership with James Gregory (1753–1821), but, though successful, preferred literature and philosophy. After twice failing to gain a professorship in the university, he was invited, during an illness of Dugald Stewart in the session of 1808-1809, to act as his substitute, and during the following session he undertook much of Stewart's work. The students received him with enthusiasm, due partly to his splendid rhetoric and partly to the novelty and ingenuity of his views. In 1810 he was appointed as colleague to Stewart, a position which he held for the rest of his life. He wrote his lectures at high pressure, and devoted much time to the editing and publication of the numerous poems which he had written at various times during his life. He was also preparing an abstract of his lectures as a handbook for his class. His health, never strong, gave way under the strain of his work. He was advised to take a trip to London, where he died.