Briefly: Being is eternal and eternally identical to itself, but the entities – the things that are perceptible – become; that is, they change form, colour and substance. Therefore, a question arises: how is it possible to harmonise the Truth of lògos (‘logic’) with the truth that our senses experience in our everyday life? The Parmenidean answer was clear: our senses are deceivers, and what they perceive is not Truth but opinion (dòxa). Being is eternal and unchangeable, hence everything that changes (phenomena) must be illusory – it cannot be Being or not-Being (which by definition it is not). This position, if not contradictory is very unstable, could not satisfy the following philosophers.

     Plato performs the so-called ‘patricide’, i.e. he denies the Parmenidean position in an attempt to ‘save the phenomena‘ from the risk of falling back into the dimension of non-Being. In the article dedicated to this philosopher, we already talked about the five ‘Supreme Genders‘ (Being, Stillness, Motion, Same and Other) and the relative and absolute non-Being, but it is now a question of understanding the transition from the absoluteness of Being to the many perceptible entities.
Socrates’s pupil starts precisely from the analysis of the Socratic questions about universals; for example: “what is Good?”, “what is Justice?”. If human beings can attempt to answer such questions, it means that they understand them, and that therefore ‘things’ such as ‘Good’ or ‘Justice’ exist in the intellect – thus they are rationally comprehensible. As absolutes – i.e. dissolved from the perceptible world – they are defined by Plato as ‘Ideas‘. But what is an ‘idea’ according to Plato? It can be defined as the intelligibility of Being itself, or, in other words, its being transparent to thought. The field of Being is divided in two worlds: the world of Ideas which is intellectually comprehensible; and, the world of entities which is accomplished through our senses. The entities, however, owe their intelligibility and their being what they are to the fact that they are ‘images‘ of Ideas. In what is defined as Hyperuranium we find pure Ideas – not materially formed – which have almost the same attributes as the Parmenidean Being: universal, always identical to themselves, necessary, eternal, immutable; but they are also immaterial, transcendent, only intellectually and qualitatively superior to material entities. These specific characteristics are a negationis definition of Ideas, i.e. they determine their difference with respect to perceptible entities. But they have a close relationship with the latter: in fact, perceptible entities are imperfect images of Ideas, and have two kinds of relationships with them: imitation (mìmesis) and participation (mèthexis). We could propose a simplification through the following scheme: in the Hyperuranium there are pure Ideas (the idea of Justice, the idea of Good, the idea of ‘table’, the idea of ‘chair’ etc.), and in the world there are entities (good things, right things, tables and chairs). Let’s take a perceptible table: it is a table because it imitates the Idea of a table and exists (it is an entity) because it participates in the Idea of a table. Every table that exists in the perceptible world is a ‘table’ mainly because it imitates and participates the Idea of a table, but because imitation and participation are always imperfect (otherwise there would be an identity between the perceptible table and its Idea) there are tables that are different in shape, colour and material. One could say that what defines and unites all the objects we call ‘table’ is their relationship with the Idea of a table.
One must bear in mind that for Plato (and for almost all ancient philosophers) it is a matter of principle that the axiom ex nihilo nihil fit (nothing comes from Nothingness), in fact the matter (chòra) that composes the perceptible entities is eternal and shapless; the matter, however, has in its highest degree the capability of being the hotbed of shapes, and can take on, through mimetic relationship, the shapes of Ideas. However, matter is precisely part of the material dimension, whereas Ideas are part of the intelligible dimension. How can they possibly relate to each other? A manipulative intelligence is required, one that has the ability to access the ideal world and manipulate inert matter, persuading it to imitate Ideas. This task falls to the Demiurge, who is the conscious cause of the participation of perceptible entities and Ideas. In Plato’s late philosophy, the emphasis is placed on the characteristics of proportion, measure and limit, in a word of order (kòsmos) to which Being tends towards, that is, its harmonious and ordered structure. In this sense multiplicity is resolved towards unity, and this also happens in the intelligible realm of Ideas. Manifesting his primary intent derived from his teacher Socrates, that is, to understand philosophy as an ethical path that can show mankind how to ‘live well’ in order to ‘become happy’, Plato places the idea of Good at the top of the intelligible world. Therefore, the idea of Good is not one mere idea among the others, but it is the true cause of other ideas: just as the Sun (light) is the cause of the knowability and existence of the perceptible world, so the Good is the cause of knowability and existence of the intelligible world; this to the point that in Platonic writings Socrates says in an obscure and enigmatic way that the Idea of Good is the only one superior to Being, both in dignity and power – and in recent writings it is increasingly referred to as the One.
The Platonic movement can therefore be summarised as a descendant of existence from the universal to the singular: only the universal (Ideas) are proper substances (i.e. they exist in themselves), while the singulars (the perceptible entities) exist only by borrowing existence from the universals in which they participate.

Starting from the criticism of Plato’s ontology, Aristotle practically turns his teacher’s position around. In fact, he notes that a substance (hypokéimenon) is by definition what exists in itself; that is, it does not derive its existence from anything other than itself. A universal, on the other hand, receives its existence from its incarnation in a singular, that is to say, in a specific material. In order to understand this concept, we will take the colour ‘red’ as a universal example. It does not concretely exist, except as a ‘predicate’ or ‘attribute’ of a particular entity: you can find a red vase, a red flower, red paint, a red tempera, etc.; but it is impossible to experience ‘red’ in its universality, that is to say, if not as being an attribute of something. In this sense for Aristotle, since ‘red’ exists thanks to the connection to predication, it cannot be substance. In the same way the idea of a table exists only as a predicate of all the entities that are tables and of which we can experience in the world. Substance, therefore, has two characteristics: ontological independence (its subsistence depends only on itself) and determinacy (it is something determined but not universal).

Furthermore, substance has the capability of being subjected to predication, and this is well understood from the analysis of the Greek term: hypokèimenon (sub-stantia, sub-jectum): ‘what lies beneath’, and therefore what sustains the predicate (e.g. ‘the short, red, chipped glass’: ‘glass’ is the substance that sustains the predicables of ‘being red’, ‘shirt’ and ‘chipped’). For this Stagiritis, the determinacy means that the substance is always a ‘tòdetì‘, i.e. a ‘this here’ – something that can be the object of a deixis, something that can be indicated. Having said that, how can we concretely define a substance? It is a sìnolo (union) of matter and form. Matter is eternal but indeterminate, indefinite, and acquires its determinacy through the form it embodies. However, if the entities are a union of matter and form that can be dissolved, the substance is in flux and corruptible, and it would seem necessary to reintroduce the concept of non-Being that Parmenides and Plato had struggled so hard to eliminate from the field of Being. Aristotle has an original solution for an old problem; it is not necessary to introduce two types of non-Being, but two types of Being: potentiality and actuality. Let us see an example: take a seed; it is a seed in action, but in power it is a tree. The appearance of the tree is not the non-Being of the seed, just as reciprocally the presence of the seed is not the non-Being of the tree. Seed and tree are both present in the seed, but the first is in action, and the second in power (the seed already has in itself the potential of being a tree).
We shall omit here, out of necessity of space, the problem of Aristotle’s categories and the need to reintroduce a supersensitive plane to explain the permanence of the form. What we will be explained now is the introduction of unity.

If substance is sìnolo of matter and form, then it is necessarily subject to motion and flux (because matter is always in power, i.e. always essentially destined to change). But in order to explain the sìnolo it is necessary there be something imperceptible and unchangeable; that is why Aristotle introduces the eternity of motion. First of all, Aristotle contests the idea that movement may have had a beginning: everything that moves is moved by something else, by a mover. If in the beginning there was no movement, and later a mover that either did not exist or did not move. If it did not exist it must have been created, but this would presuppose a creative movement, and therefore a movement. If it did not move, there can only be two reasons: either something prevented it from moving, but this impediment was in turn a movement (even if of a contrary nature), or it had to be set in motion, but what set it in motion had to be a movement in turn. In short, for every possibility it would be necessary to create an infinite chain of causes, but the principle that regressus in infinitum is not an acceptable explanation applies. Therefore, it can be deduced that movement is infinite as well as time (it is composed of instants, but there cannot be an instant before time or an instant before the last one because they would be eternal and therefore there would never be an instant after the first, nor an instant before the last).
Now, the cause of eternal movement must in turn be eternal (producing movement for eternity), and therefore there must be a first and eternal mover. This also cannot be moved by any other mover, and everything moved is moved not by itself but by something else. Therefore, it is deduced that there is a Prime or Unmoved Mover, which moves everything but is moved by nothing. Hence, a spontaneous question arises: how can immobility produce movement? First of all, it is precisely because it is immobile, this mover cannot have any potential: it is pure act and pure form; moreover, it is substantial, because it finds the cause of its being only in itself. Potentiality can also be defined as the drive for its own realisation: everything wants to fulfil (actualise) all its potentialities, since remaining in power is a lack of self-fulfilment.

The motionless mover is a pure act, therefore a totally unfolded actuality; all things desire to be in turn a totally unfolded act, and therefore yearn for it without ever being able to reach it (since they are material, and therefore by their very nature potential). As a totally unfolded act the motionless motor attracts towards itself the substances that yearn for its own condition and is the final cause of eternal movement.
This mover is the final cause of all things, it is perfection inasmuch a total unfolding of its own actuality, it is necessary because it cannot be anything other than what it is, and it cannot be the object of perceptible experience because it is immaterial; it is clear that for Aristotle we are talking about God. However, there is a distinction to be made: there are many movements in the universe and each of them has its own mover: there are fifty-five of them. Then again, what is unique is the first principle.
The immovable mover is life as it ‘gives’ life to everything as the final cause of its movement. It is not creator but activity (pràxis); and since it is immaterial and immobile its own activity is thought. Yet, thought must have an object, but since the unmovable mover is a totally unfolded act (current totality of all possibilities) the object of its thought is itself: its activity is thought of thought (noésis noéseos) – intelligence and intelligible, here, coincide.
But what does the first principle – God – thinks of himself actually mean? God is pure perfection, total unfolding of all actuality; by thinking of himself God thinks of everything as currently accomplished in the full splendour of his total self-fulfilment. Aristotle’s God thinks of himself is the thought of the total and pure fulfilment of the Whole.