It is sometimes thought that the medieval scholars did not consider that experience and observation were of any value in the search for truth, and that therefore there could have been no development of science. In an article on "Science at the Medieval Universities"[2] I made a series of quotations from the two great scientific scholars of the thirteenth century, Albertus Magnus and Roger Bacon, with regard to the question of the relative value of authority and observation in all that relates to physical science. Stronger expressions in commendation of observation and experiment as the only real sources of knowledge in such matters could scarcely be found in any modern scientist. In Albert's tenth book of his "Summa," in which he catalogues and describes all the trees, plants, and herbs known in his time, he declares: "All that is here set down is the result of our own experience, or has been borrowed from authors whom we know to have written what their personal experience has confirmed; for in these matters experience alone can be of certainty." In his impressive Latin phrase, _experimentum solum certificat in talibus_. With regard to the study of nature in general he was quite emphatic. He was a theologian as well as a scientist, yet in his treatise on "The Heavens and the Earth," he declared that: "In studying nature we have not to inquire how God the Creator may, as He freely wills, use His creatures to work miracles, and thereby show forth His power. We have rather to inquire what nature with its immanent causes can naturally bring to pass."