James Boswell's uncle John was a member of a sect known as the Glasites.
Life with Boswell:
Stewart visited Patrick Heron's Kirroughtrie during Boswell's stay at the place on October 1, 1762. Boswell described Stewart as "a man of sense and business". Stewart seems to have been a relation of Heron's, his father's second wife being a Heron herself.
Note 1: According to Journal of my Jaunt, 1762.
Note 2: Namier, L. & Brooke, J. (1964). The House of Commons 1754-1790. (Available via AbeBooks.com)