Aristotle not only deals positively and experimentally with the natural order and with man so far as he is a part of this order, but he deals in a similar fashion with a side of man that the modern positivist often overlooks. Like all the great Greeks Aristotle recognizes that man is the creature of two laws: he has an ordinary or natural self of impulse and desire and a human self that is known practically as a power of control over impulse and desire. If man is to become human he must not let impulse and desire run wild, but must oppose to everything excessive in his ordinary self, whether in thought or deed or emotion, the law of measure. This insistence on restraint and proportion is rightly taken to be of the essence not merely of the Greek spirit but of the classical spirit in general. The norm or standard that is to set bounds to the ordinary self is got at by different types of classicists in different ways and described variously: for example, as the human law, or the better self, or reason (a word to be discussed more fully later), or nature. Thus when Boileau says, “Let nature be your only study,” he does not mean outer nature, nor again the nature of this or that individual, but representative human nature. Having decided what is normal either for man or some particular class of men the classicist takes this normal “nature” for his model and proceeds to imitate it. Whatever accords with the model he has thus set up he pronounces natural or probable, whatever on the other hand departs too far from what he conceives to be the normal type or the normal sequence of cause and effect he holds to be “improbable” and unnatural or even, if it attains an extreme of abnormality, “monstrous.” Whatever in conduct or character is duly restrained and proportionate with reference to the model is said to observe decorum. Probability and decorum are identical in some of their aspects and closely related in all.[30] To recapitulate, a general nature, a core of normal experience, is affirmed by all classicists. From this central affirmation derives the doctrine of imitation, and from imitation in turn the doctrines of probability and decorum.