He believed in the existence of the four elements, earth, air, fire, and water,[69] and that they were the visible manifestations of one underlying matter.[70] They were not mutually exclusive but “all elements existed in all”, and it was possible for one element to be transmuted into another. Their properties were not invariable, but as a rule fire is spoken of as hot and dry; air, hot and wet; water, wet and cold; earth, cold and dry. It will be observed that each successive pair of elements had a common quality: thus fire and air shared the quality of ‘hot’; air and water, that of ‘wet’; water and earth, that of ‘cold’; earth and fire, that of ‘dry’. It was by the aid of these common qualities, which served as means, that the elements could be more easily thought of as passing into each other.[71] [69] For a clear account of the theory of the four elements in medieval thought see _Les Quatre Elements_, J. Leminne in _Mémoires couronées par l’Académie Royale de Belgique_, v. 65, Bruxelles, 1903. [70] _Etym._, 13, 3. _Cf. D. N. R._, 11. [71] The theory of atoms is also stated by Isidore. See p. 235. It is not used, however, and is not fully stated. The part played in the theory by atoms of different sizes is not mentioned, and although “the void” is mentioned, its importance is not brought out. It should be remarked that the general idea is the same as that of modern chemistry in so far as it assumes that there are elements and attributes properties to them. The difference is that the modern chemist insists that the properties shall be fixed for each element, while Isidore has no consciousness of such a necessity. For instance, in a chapter of _De Natura Rerum_ he attributes two separate sets of properties to the four elements, without realizing at all the confusion of such a procedure. Again, from the point of view of the best ancient conception of the four elements, Isidore is equally at fault. For Aristotle the names given to them had been merely labels. He perceived in the natural world two significant sets of opposing qualities, namely, hot and cold, wet and dry. These sets of opposing qualities interpenetrated one another: the result was four possible combinations, namely, hot and dry, hot and wet, cold and wet, cold and dry. His elements designated merely these combinations and were nothing more than conventional names for them. Isidore, however, took the names of the elements in a literal sense.[72] The label itself had become important, while what stood behind it and gave it its value was regarded as almost meaningless. What has happened here is typical of the whole development of ancient thought down to Isidore’s time. [72] See Art. “Chemistry,” _Encyclopedia Britannica_, 11th edition.