Aristotle agreed with his predecessor Empedocles that Fire, Water, Air, and Earth are the four elements, distinguished from compounds like flesh and bone but like them in being, contrary to the atomists’ view, homoeomerous: “any part of such a compound is the same as the whole, just as any part of water is water” (De generatione et corruptione (DG) I.10, 328 a 10f.). Rather than simply assuming the elements were four in number, however, he claimed to prove this from more fundamental principles in connection with a new theory of mixture, involving combination by the interaction of the original substances and developed from the same fundamental principles. These interactions arise in virtue of the mutual powers and susceptibilities conferred by properties which can be reduced to grades of warmth and humidity (DG II.2). There are maximal and minimal “contrary extremes” (DG II.8, 335 ^a 8) of each of these two fundamental scales, hot and cold being the extremes of warmth, moist and dry those of humidity, and elements are substances with these extremal properties. Since “it is impossible for the same thing to be hot and cold, or moist and dry ... Fire is hot and dry, whereas Air is hot and moist ...; and Water is cold and moist, while Earth is cold and dry” (DG II.3, 330 ^a 30-330 ^b 5). But combination will occur on mixing any two substances with different degrees of these two fundamental scales, and original substances are only distinguished as elements by virtue of their extreme degrees of warmth and humidity. The theory of mixing contains no restriction confining the original ingredients to elements as just defined. So when he goes on to agree with “all who make the simple bodies elements” (330 ^b 7), it is not clear what the simplicity of the elements amounts to since the notion of being an original ingredient is not characteristic of elements and atomic conceptions have been rejected. A definition in terms of simplicity is given in De caelo III.3, however, where Aristotle says “An element, we take it, is a body into which other bodies may be analysed, present in them potentially or in actuality (which of these is still disputable), and not itself divisible into bodies different in form. That, or something like it, is what all men in every case mean by element” (302 ^a 15ff.). An account of simplicity in terms of possessing “a principle of movement in their own nature” (268 ^b 28) is elaborated earlier.