Thomas Harper is one of the great forgotten Neo-Scholastic writers of the nineteenth century. I discussed his wonderful little book The Immaculate Conception in a blog post . He is especially notable for his unusually rigorous and thorough treatment of abstract topics in metaphysics, in works such as the massive three-volume The Metaphysics of the School. Harper will sometimes interrupt a sustained exercise in abstract reasoning with a non-technical aside, as he does in the course of discussing the metaphysics of truth in Volume I. He there offers (at pp. 461-466) a commentary on Francis Bacon’s “idols of the mind” which is even more relevant now than it was in Harper’s day.