He developed a unique brand of materialist philosophy during his travels that combines spinozistic pantheism with yogic concepts of a single indivisible consciousness. Stewart began expressing his opinions in public with the publication of his book Travels through the most interesting parts of the Globe (London, 1790).
The Opus Maximum (London, 1803) and the epic verse poem The Revelation of Nature (New York, 1795), two philosophical works by Stewart that were released over the course of the next three decades.
Stewart's works are characterised by a naive arrogance, frequently asserting that their author is the 'only child of nature' to have ever lived. Stewart was portrayed in a pastel by James Sharples, a portrait artist for George Washington, in 1796 for a collection of portraits.