Vision and Certitude in the Age of Ockham. Optics, epistemology and the foundations of Semantics, 1250–1345. By Katherine H. Tachau. (Studien und Texte zur Geistesgeschichte des Mittelalters, xxn.) Pp. xxii + 428. Leiden: Brill, 1987. Gld. 170/$77.25. 90 04 08552 1; 0169 8125