Over the next three decades Stewart wrote prolifically, publishing nearly thirty philosophical works, including The Opus Maximum (London, 1803) and the long verse-poem The Revelation of Nature (New York, 1795).

[3] During his journeys, he developed a unique system of materialist philosophy which combines elements of Spinozistic pantheism with yogic notions of a single indissoluble consciousness.

Historian David Fairer has written that "Stewart expounds what might be described as a panbiomorphic universe (it deserves an entirely new term just for itself), in which human identity is no different in category from a wave, flame, or wind, having an entirely modal existence.".

Stewart declared that if he were about to die, these should be his last words: "The only measure to save mankind and all sensitive life is to educate the judgment of man and not the memory, that he may be able through reflection to calculate the golden mean of good and evil".

In 1792, while residing in Paris in the weeks following the September Massacres, he made the acquaintance of the young Romantic poet William Wordsworth, who later concurred with De Quincey in describing Stewart as the most eloquent man on the subject of Nature that either had ever met.