Burthogge had no great reputation in his own day, and was almost entirely forgotten afterwards, till recent historians drew attention to his merits. His chief work, An Essay upon Reason and the Nature of Spirits, was published in 1694 and dedicated to Locke “as to a person … acknowledged by all the learned world for one of the greatest masters of reason.” But he cannot be counted either as a follower or as a critic of Locke. His characteristic doctrines had been expressed in an earlier work, Organum vetus et novum, published in 1678. He had come into contact independently with the Cartesian reform; he was acquainted (though he did not sympathise) with the work of Malebranche; and he may have been influenced directly by Geulincx, who was lecturing in the university of Leyden when Burthogge studied medicine there and, in 1662, graduated M.D. Burthogge’s object was to reconcile the experimental or mechanical with the scholastic method. His most striking doctrine, however, concerns the subjective factor in knowledge, and this led to his assertion of the relativity of all knowledge. What Descartes and Locke had said of the secondary qualities is generalised. The understanding apprehends things only by its own notions: these are to it what colours are to the eye or sounds to the ear; whole and part, substance and accident, cause and effect are but “entities of reason conceived within the mind,” and “have no more of any real true existence without it, than colours have without the eye, or sounds without the ear.” With this radical doctrine of relativity, Burthogge combined a neoplatonic metaphysic. He held that there is one spirit that actuates and acts in all, in men as well as in nature, and that the spirit of nature is not (as Henry More taught) an incorporeal substance, but simply the “plastic faculty” of the spirit of God.