Reflecting his varied professional experiences, Forsyth's chief works include Principles and Practice of Agriculture (2 vols. 1804), The Principles of Moral Science (vol. i. 1805), Political Fragments (1830), and Observations on the Book of Genesis (1846). However, his best-known work is The Beauties of Scotland (5 vols. 1805–8), which still maintains some popularity today, due in part to the many engravings which it contains of Scottish towns and places of interest.
At age seventy-six, Forsyth – reflecting his continuing loyalty to the Church – published a pamphlet entitled Remarks on the Church of Scotland, &c (1843). This work was reviewed critically by Hugh Miller, then editor of the Witness, who ridiculed Forsyth's remarks in the pamphlet as well as some of his past speculations on philosophy (several of which bear similarities to commonly accepted current views). For example, Forsyth once noted: "Whatever has no tendency to improvement will gradually pass away and disappear for ever", hinting at the now-commonplace concept of the survival of the fittest. In addition, Forsyth wrote: "Let it never be forgotten then for whom immortality is reserved. It is appointed as the portion of those who are worthy of it, and they shall enjoy it as a natural consequence of their worth." This view seems to parallel the doctrine of conditional immortality now held by many Christians. At the time, however, Hugh Miller said ironically of these views: "It was reserved for this man of high philosophic intellect to discover, early in the present century, that, though there are some souls that live for ever, the great bulk of souls are as mortal as the bodies to which they are united, and perish immediately after, like the souls of brutes."
Forsyth died in Edinburgh on 30 September 1845.[4]