Aristotle

“ Necessity itself has two aspects, one kind being that which obeys nature or a natural impulse, another that which acts under compulsion and contrary to its own impulse. Thus a stone moves both upwards and downwards ‘of necessity,’ though not owing to the same kind of necessary. ”

“ As then a thing which has a final cause and is necessary can exist, so also such a thing can come into existence. For instance, suppose thunder to arise both because there must necessarily be hissings and roarings when fire is quenched and also, as the Pythagoreans hold, in order to menace those in Tartarus and inspire them with dread. Most instances are of this kind, especially things which are or have been produced by natural laws, for nature works in some cases with a definite purpose, in others of necessity. ”

“ All these causes may serve as middle terms. The material cause cannot be demonstrated by means of one premise only, there must be at least two, and that can only happen when one middle term is added. When this is done a conclusion must necessarily follow. This may be made clear from the following example; ‘Why is the angle in a semicircle a right angle? or, under what conditions is it a right angle?’
Let A be right angle, B the half of two right angles, C the angle in a semicircle. Now B is the cause of A, right angle, being predicable of C, the angle in a semicircle ”