Heat and cold, wetness and dryness, are the four principal properties of the four elements, and they are accidents inseparable from them, and two of these properties are active, i.e., heat and cold. The reason why they are designated active qualities is that when we touch them, they make known to us then immediately at that very moment their own essence, for when we touch fire, it makes known to us then the essence of its heat. So, when we touch hoar frost it makes known to us then immediately at that same moment the essence of its coldness. The reason why we feel those immoderate things is that we have a moderate nature.
The remaining two properties are passive, and the reason why they are designated passive qualities here is because they do not make known their powers when touched, for when we touch a wet object or a dry object we do not feel its wetness or its dryness suddenly as we feel the heat of the fire or the
p.139cold of the hoarfrost. That is why some of those properties are said to be active and some passive. Every body from the sphere of fire downwards is compounded of these (qualities) and they themselves (the bodies) are simple, even though they are said to be compounded of their own properties as hot, dry fire is a compound of those two properties i.e., of heat and dryness, since it is those two properties that preponderate in fire. And air is a compound of dryness and dampness, because it is they that preponderate in it and thus is the preponderance of the other two elements with regard to their own properties.
Although these four elements are compounded of the principal properties, they are termed simple in comparison with the elemental bodies that are compounded of themselves. Thus these four are both compound and simple. Simple compared to every object that is compounded of themselves. Compound compared to the prime qualities which are essential. Thus it is fitting for a simple body to have a simple motion, and for a compound body to have a compound motion.
And it is clear, that every body in which heat preponderates, moves from the centre upwards; and every body in which cold preponderates, moves in the direction of the centre. It is the heat that causes lightness in the natural bodies and it is the cold that causes heaviness and it is the dryness that causes rapidity of movement towards their natural place in light bodies. In the same way dampness causes slowness of motion in the bodies in which it is. From these statements we conclude that it is the nature of every one of those elements to remain in its own natural position in which is the end of its own motion, because if one of those elements were displaced by force from its own natural position, the nature of it would draw it again to the same position.