Bacon insisted that the four great grounds of human ignorance 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.'" Prof. Henry Morley suggested that: "No part of that ground has yet been cut away from beneath the feet of students, although six centuries have passed. We still make sheepwalks of second, third, and fourth, and fifth 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." It used to be the custom to make little of the medieval scientists because of their reverence for Aristotle. Generations who knew little about Aristotle, especially those of the seventeenth and eighteenth centuries, were inclined to despise preceding generations who had thought so much of him. We have come to know more about Aristotle in our own time, however, and as a consequence have learned to appreciate better medieval respect for him. Very probably at the present moment there would be almost unanimous agreement of scholars in the opinion that Aristotle's was the greatest mind humanity has ever had. This is true not only because of his profound intellectual penetration, but above all because of the comprehensiveness of his intelligence. For depth and breadth of mental view on a multiplicity of subjects, Aristotle has never been excelled and has but very few rivals. The admiration of the Middle Ages for him, instead of being derogatory in any way to the judgment of the men of the time, or indicating any lack of critical appreciation, rather furnishes good reasons for high estimation of both these intellectual modes of the medieval mind. Proper appreciation of what is best is a much more difficult task than condemnation of what is less worthy of regard. It is the difference between constructive and destructive criticism. Medieval appreciation of Aristotle, then, constitutes rather a good reason for admiration of them than for depreciation of their critical faculty; and yet they never carried respect and reverence to unthinking worship, much less slavish adoration. Albertus Magnus, for instance, said: "Whoever believes that Aristotle was a God must also believe that he never erred; but if we believe that Aristotle was a man, then doubtless he was liable to err just as we are." We have a number of direct contradictions of Aristotle from Albert. A well-known one is that with regard to Aristotle's assertion that lunar rainbows appeared only twice in fifty years. Albert