Unfortunately for his reputation he had a weak side.

Enlightened though he was, he was a firm believer in witchcraft, and he is chiefly remembered not as an admirer of Descartes and Bacon, and a champion of the Royal Society, but as the author of Saducismus Triumphatus, a monument of superstition, which probably contributed to check the gradual growth of disbelief in witches and apparitions.
His Plus ultra is a review of modern improvements of useful knowledge.
It is confined to mathematics and science, in accordance with its purpose of justifying the Royal Society; and the discoveries of the past sixty years enable the author to present a far more imposing picture of modern scientific progress than was possible for Bodin or Bacon.
[Footnote: Bacon indeed could have made out a more impressive picture of the new age if he had studied mathematics and taken the pains to master the evidence which was revolutionising astronomy.

Glanvill had the advantage of comprehending the importance of mathematics for the advance of physical science.] He had absorbed Bacon's doctrine of utility.

His spirit is displayed in the remark that more gratitude is due to the unknown inventor of the mariners' compass "than to a thousand Alexanders and Caesars, or to ten times the number of Aristotles.

And he really did more for the increase of knowledge and the advantage of the world by this one experiment than the numerous subtile disputers that have lived ever since the erection of the school of talking." Glanvill, however, in his complacency with what has already been accomplished, is not misled into over-estimating its importance.