Another mechanical explanation was based upon a postulated tendency of atoms to move into a vacuum rather than upon the latter's non-existence. Lucretius restated this Epicurean explanation in his _De rerum natura_.[14] Atoms from the loadstone push away the air and tend to cause a vacuum to form outside the loadstone. The structure of iron is such that it, unlike other materials, can be pushed into this empty space by the thronging atoms of air beyond it. [14] Lucretius, _De rerum natura_, translated by W. H. D. Rouse, Loeb Classical Library, London, 1924, bk. VI, lines 998-1041. Galen[15] returned to a quasi-animistic solution in his denial of Epicurus' argument, which he stated somewhat differently from Lucretius. One can infer that Galen held that all things have, to a greater or lesser degree, a sympathetic faculty of attracting its specific, or proper, quality to itself.[16] The loadstone is only an inanimate example of what one finds in nutritive organs in organic beings. [15] Galen, _On the natural faculties_, translated by A. S. Brock, Loeb Classical Library, London, 1916, bk. 1 and bk. 3. A view similar to this appeared in Plato, _Timaeus_, 81 (see footnote 13). [16] This same concept was to reappear in the Middle Ages as the _inclinatio ad simile_. One of the few writers whose explanations of the loadstone Gilbert mentioned with approval is St. Thomas Aquinas. Although the medieval scholastic philosophy of St. Thomas seems foreign to our way of thinking, it formed a background to many of Gilbert's concepts, as well as to those of his predecessors, and it will assist our discussion to consider briefly Thomist philosophy and to make its terminology explicit at this point.[17] [17] The background for much of the following was derived from Annaliese Maier, _An der Grenze von Scholastik und Naturwissenchaft_, ed 2, Rome, 1952. In scholastic philosophy, all beings and substances are a coalescence of inchoate matter and enacting form. Form is that which gives being to matter and which is responsible for the "virtus" or power to cause change, since matter in itself is inert. Moreover, forms can be grasped intellectually, whence the nature of a being or a substance can be known. Any explanation of phenomena has to be based upon these innate natures, for only if the nature of a substance is known can its properties be understood. Inanimate natures are determined by observation, abstraction, and induction, or by classification.[18] [18] St. Thomas' epistemology for the natural inanimate world was based upon Aristotle's dictum: that which is in the mind was in the senses first. The nature of a substance is causally prior to its properties; while the definition of the nature is logically prior to these properties. Thus, what we call the theory of a substance is expressed in its definition, and its properties can be deduced from this definition.