Indeed, it seems very clear that the whole trend of thought among the great teachers of the time was away from the acceptance of scientific conclusions on authority unless there was good evidence for them available. They were quite as impatient as the scientists of our time with the constant putting forward of Aristotle as if that settled a scientific question. Roger Bacon wanted the Pope to forbid the study of Aristotle because his works were leading men astray from the study of science, his authority being looked upon as so great that men did not think for themselves but accepted his assertions. Smaller men are always prone to do this, and indeed it constitutes one of the difficulties in the way of advance in scientific knowledge at all times, as Roger Bacon himself pointed out. These are the sort of expressions that are to be expected from Friar Bacon from what we know of other parts of his work. His "Opus Tertium" was written at the request of Pope Clement IV, because the Pope had heard many interesting accounts of what the great thirteenth-century teacher and experimenter was doing at the University of Oxford, and wished to learn for himself the details of his work. Bacon starts out with the principle that there are four grounds of human ignorance. These are, "first, trust in inadequate authority; second, that force of custom which leads men to accept without properly questioning what has been accepted before their time; third, the placing of confidence in the assertions of the inexperienced; and fourth, the hiding of one's own ignorance behind the parade of superficial knowledge, so that we are afraid to say I do not know." Professor Henry Morley, a careful student of Bacon's writings, said with regard to these expressions of Bacon: No part of that ground has yet been cut away from beneath the feet of students, although six centuries have passed. We still make sheep-walks of second, third and fourth, and fiftieth hand references to authority; still we are the slaves of habit, still we are found following too frequently the untaught crowd, still we flinch from the righteous and wholesome phrase "I do not know" and acquiesce actively in the opinion of others that we know what we appear to know. In his "Opus Majus" Bacon had previously given abundant evidence of his respect for the experimental method. There is a section of this work which bears the title "Scientia Experimentalis." In this Bacon affirms that "without experiment nothing can be adequately known. An argument may prove the correctness of a theory, but does not give the certitude necessary to remove all doubt, nor will the mind repose in the clear view of truth unless it finds its way by means of experiment." To this he later added in his "Opus Tertium": "The strongest argument proves nothing so long as the conclusions are not verified by experience. Experimental science is the queen of sciences, and the goal of all speculation."