Until the thirteenth century, dialectic and theology were not completely severed from _belles lettres_. The Platonic-Augustinian theology of the twelfth century had been idealizing and imaginative, not to say poetical. Such an interesting exponent of it as Hugo of St. Victor appears as a literary personage, despite his stinted advocacy of classical study. One notes that for his time the chief single source of physical knowledge was the Latin version of the _Timaeus_, certainly not a prosaic composition. Thus, for the twelfth century, an effective cause of the continuance of the study of letters lay herein: whatever branch of natural knowledge might allure the student, he could not draw it bodily from a serious but unliterary repository, like the _Physics_ or _De animalibus_ of Aristotle, which were not yet available; he must follow his bent through the writings of various Latin poets as well as prose-writers. In fine, the sources of profane knowledge open to the twelfth century were literary in their nature, and might form part of the literature which would be read by a student of grammar or rhetoric. One sees this in John of Salisbury. There may have been a few men who knew more than he did of some particular topic. But his range and readiness of knowledge were unique. And it is evident from his writings that his knowledge (except in logic) had no special or scientific source, but was derived from a promiscuous reading of Latin literature. As a result, he is himself a literary man. One may say much the same of his younger contemporary, Alanus de Insulis.[172] He too has gathered knowledge from literary sources, and he himself is one of the best Latin poets of the Middle Ages. Another extremely poetic philosopher was Bernard Silvestris, the interpreter of Virgil. His _De mundi unitate_ is a Pantheistic exposition of the Universe; it is also a poem; and incidentally it affords another illustration of the general fact, that before the works of Aristotle were made known and expounded in the thirteenth century, all kinds of natural and quasi-philosophic knowledge were drawn from a variety of writings, some of them poor enough from any point of view, but none of them distinctly scientific and unliterary, like the works of Aristotle. Formal logic or dialectic, as cultivated by Abaelard for example, appears as an exception. It had been specialized and more scientifically treated than any branch of substantial knowledge; for indeed it was based on the logical treatises of Aristotle, most of which were in use before Abaelard’s death, and all of which were known to Thierry of Chartres and John of Salisbury.[173]