 The importance of measure in a literal sense becomes more explicit, however, in the Philebus, the dialogue that is concerned with the question of whether pleasure or knowledge constitutes the human good. In that dialogue, number (arithmos), measure (metron), and limit (peras) play a crucial role at various points of the discussion, and the Philebus is the dialogue where Plato requires that numerical precision must be observed in the application of the ‘divine gift’ of dialectical procedure by the method of collection and division (16c–17a). The dialectician must know precisely how many species and subspecies a certain genus contains; otherwise he has no claim to any kind of expertise. Despite this emphasis on precision and on the need to determine the numerical ‘limit’ in every science, Socrates does not provide the envisaged kind of numerically complete division of the two contenders for the rank of the best state of the human soul – pleasure and knowledge – envisaged by the use of collection and division. This is because Socrates suddenly remembers that neither of the two contenders suffices in itself for the good life, and that a mixture of the two is preferable. To explain the nature of this mixture, Socrates introduces a fourfold division of all beings (23c–27c), a division that uses the categories of ‘limit’ and ‘measure’ in a different way than in the ‘divine method of dialectic’. As he now states, all beings belong in one of four classes – namely (1) limit (peras), (2) the unlimited (apeiron), (3) the mixture (meixis) of limit and the unlimited, or (4) the cause (aitia) of such a mixture. As the subsequent explications concerning the four classes show, the unlimited comprises all those things that have no exact grade or measure in themselves, such as the hotter and colder, the faster and slower. Although at first the examples are confined to relative terms, the class of the unlimited is then extended to things like hot and cold, dry and moist, fast and slow, and even heat and frost. Mixture takes place when such qualities take on a definite quantity (poson) or due measure (metrion) that limits their variation. That only measured entities qualify as mixtures is not only suggested by the examples Socrates refers to (health, strength, beauty, music, and the seasons), but by his assertion much later in the dialogue that a mixture without due measure or proportion does not deserve its name (64d–e): “[it] will necessarily corrupt its ingredients and most of all itself. For there would be no blending in such cases at all, but really an unconnected medley, the ruin of whatever happens to be contained in it.” The upshot of this discussion is that all stable entities (mixtures) represent a harmonious equilibrium of their otherwise unlimited ingredients. Since indeterminate elements usually turn up in pairs of opposites, the right limit in each case is the right proportion necessary for their balance. In the case of health, there must be the right balance between the hot and the cold, the dry and the moist. The cause of the proper proportion for each mixture turns out to be reason; it is the only member of the fourth class. As Socrates indicates, divine reason is the ultimate source of all that is good and harmonious in the universe, while human reason is but a poor copy (26e–27c; 28a–30e).