We can come across the idea of the mean in practically all the branches of Aristotle's doctrine, from ontology and metaphysics to ethics and politics. Aristotle argues that one can detect the mean "in everything continuous and divisible" if "there is excess and deficiency." And the basis of this continuity is a motion, "for motion is continuous and action is motion" (Eud. Eth II.3.1220b 21-35). In other words, the mean is characteristic of something continuous, existing in the form of arithmetical progression (from deficiency to excess), as well  as of something dynamic, changeable and complex. In ethics, it is a virtue; in a syllogism — a middle term; in a state — a middle class; in time — a "now"; in man — a soul. But for every field of human activity mentioned, its mean is the only static and not developing point — a point of stable equilibrium in balancing between "excess" and "deficiency."
The intricacy of this term (the mean) is deepened by some symptomatic inconsistency between the principles of the discourse proclaimed by Aristotle himself and the real foundation of his philosophy. He repeatedly affirmed the principle of the excluded middle and the logical impossibility for something having contrary characteristics (for instance, Metaphysics 1011b 20). However, in his own reasoning about the Mind (Nous), the knowing subject and its object become one autonomous self-subsisting "existent," "what a thing is to be per se" (to on en einai), "final good" (to ariston), "actuality" (entelechia), "first mover" (to proton kinoyn) which is itself unmovable. All these notions presuppose a kind of closure, coincidence or concurrence between the contraries — a beginning (arche), or a cause (aitia), is at the same time an end, or a goal (telos). The latter is not only a result, a final moment of any development, it is initially present from the very beginning, or even before the beginning of things and processes. Thus, a goal, constituting a limit, presents itself both as the beginning and as the final cause and substance.1 In other words, there is a hidden identity beneath the contraries.
 The notion of the mean here comes to the fore — it is in the middle, that the beginning and the end, the cause and the goal come together. That is why "... in all our inquiries we are asking either whether there is a 'middle' or what the 'middle' is: for the 'middle' here is precisely the cause, and it is the cause that we seek in all our inquiries" (Anal Post. 90a 10, tr. by G.R.G. Mure). According to J. van der Meulen, Aristotle's "mean, when it comes to the limit of penetration into a true nature of things, is the Mind in its purest form."2
Thus the mean is a structural and ontological notion — a kind of perfect, completely accomplished actual state (entelechia) through which "breathes" the Absolute and which, in its most perfect form, is the Absolute itself (Nous, Theos). The analysis of the ethical mean must be firmly based on these metaphysical principles. (2007, p. 62-64)
- 1. "Limit means (1) the last point of each thing, i.e. the first point beyond which it is not possible to find any part, and the first point within which every part is; (2) the form, whatever it may be, of a spatial magnitude or of a thing that has magnitude; (3) the end of each thing (and of this nature is that towards which the movement and the action are, not that from which they are, though sometimes it is both, that from which and that to which the movement is, i.e. the final cause); (4) the substance of each thing, and the essence of each; for this is the limit of knowledge; and if of knowledge, of the object also. Evidently, therefore, 'limit' has as many senses as 'beginning,' and yet more; for the beginning is a limit, but not every limit is a beginning" (Met. 1022a 5-10 tr., W.D. Ross).
- 2. J. van der Meulen, Aristoteles. Die Mitte in seinem Denken, Meisenheim, 1951, c. 124-25.