His health was failing during this period and he fell seriously ill in 1793. He recovered and published in 1794 ‘A Dissertation upon the Philosophy of Heat, Light and Fire’ followed by ‘An Investigation of the Principles of Knowledge and the progress of Reason, from Sense to Science and Philosophy’. In it he considers causation, space, time, the origin of society, morality, politics and education His ‘Elements of Agriculture’ was possibly written over a 40 year period and unfinished when he died in 1797. He may not have meant it to be published.
His theory on time is of particular interest in relation to his intellectual capacity to expound the enormity of time necessary for his suppositions in his Theory of the Earth. He held that there was no such thing as time, it is merely a convenient way of describing the duration of events; it continues for as long as it takes for an event or events to exit.
It is argued that these dissertations on natural philosophy, in particular his Investigation into the Principles of Knowledge (which runs to more than 3,000 pages), were part of an ambitious intellectual project to establish the grounds of rational human knowledge, influenced by his belief in deism.