Saint Thomas Aquinas’ philosophy can be regarded as the pinnacle of rational theology, both in terms of its depth and its extensive all-encompassing nature. It stems from the encounter of earlier theology – particularly Saint Augustine’s – with the ‘esoteric’ philosophy of Aristotle. While the Aristotelian texts of logic had always been available to the Western world, the ‘esoteric’ texts were carried East, and were only reunited with the corpus by Andronicus of Rhodes between 40 and 20 BC. Without going into philological reconstructions, it is sufficient to consider that ‘physical and metaphysical’ Aristotelianism definitively took place in the European cultural context during the second half of the thirteenth century, and the conception of reality therein contrasted with that of Augustine.
This premise is necessary in order to indicate some forewords to the reader that will now be offered. The conceptual complexity of the thought that will be presented here is quite significant, especially for those who have not already acquired the necessary tools for an immediate understanding of the lexicon and argumentative structures. This implies the need for a considerable clarifying effort on the part of the writer – and this is obviously his own duty – but also a commitment to attention and method on the part of the reader. For this reason, it is advisable to read, as prolegomena to this text, the articles on Aristotle’s thought that can be found in issues 4 and 5 of this magazine (i.e. “The most well-founded principle” and “Part and the whole”).
God is pure act, pure actuality totally unfolded, and every other substance participates of His being.
The text in question is an early work by St Thomas, written probably between 1252 and 1256, even though is more likely a period immediately before 1256, when ‘Doctor Angelicus’ was between 29 and 30 years old. The opusculum is entitled ‘De ente et essentia’, and presents the fundamental structure of Thomas’ ontology. The questions from which the thought arises are the following: ‘what is ens?’, ‘what is essence?‘, and the most general answer is the following: they are the first concepts of the intellect, and therefore the obligatory starting point for a rational reflection on Being. Ens is ‘id quod est’, ‘something that is’, and is composed of two constitutive principles: ‘id quod’, the ‘subject’ and ‘est’, the ‘act of existing’. According to Thomas, ens can be understood in two different ways: the first is in accordance with the ten Aristotelian categories, and the second, for everything from which an affermation can be formed. The first way is for the ens in re, that is, ens that concretely exists, while the second way is also for negations and deprivations (e.g. ‘blindness’, which is not a real ens but a deprivation of a reality, sight). It is evident that if one is looking for the essence of an ens, one must understand it in the first way. Let us now read a passage that clarifies the general way in which Thomas’ argument unfolds:
From this passage all the information necessary for this analysis can be gathered. First of all, the need to search for essence: it is the answer to the question ‘quid est?’, ‘what is it?’, or the quiddity of the thing or its ‘nature’ according to Boethius’ lexicon. Thomas writes that first of all substances are called entia, and it will be remembered that substantia is the Latin translation of the Greek term hypokeímenon, that is, ‘what “lies” underneath’, ‘what sustains’ the accidents (for example, the substance of the apple sustains its being ripe, sweet and red; the accidents are, precisely, accidental: it could very well be unripe, sour and green). Substances are divided into three orders: composite substances, simple substances and simplest/first substance.
Composite substances are the most well-known and recognisable and form the perceptible reality for human beings. They are the composition of matter and form, and therefore naturally exist. So, what is their essence? Since it is necessary for form and matter to be present together, their essence is the composition of the two. However, at the same time, since their existence is not necessary (an apple may exist, but it need not exist) their essence will be different from their being: the act of being is distinct from their essence. In addition: being (esse) is the radical and constituent act of things that are, since it is what they exist for.
On the other hand, simple substances are those that are immaterial, i.e. whose form is not imprinted on matter. These are ‘souls’ and ‘intelligences’, in which there is only the composition of form and being. Their essence cannot be identical to their act of being, otherwise the necessity of form and their multiplicity would not be explained. At the same time form cannot be the efficient cause of their being, otherwise they would be both cause and consequence, that is, their efficient cause and this would be contradictory. Therefore, their essence is only form, and their being is caused by something else, it comes from something external. It must therefore be stated that the form is potential, and the act of being is what transforms this potentiality of the form into an act. In this manner Thomas can take up the Aristotelian ‘power-act’ structure, without engaging in assuming an eternal receptive matter.
Since essence and existence are two distinct principles – and composite and simple substances cannot be the cause of their own existence – there must be something that participates in their ‘being’ i.e. the efficient cause of their existence. This first and most simple substance must be the cause of itself in the most radical sense possible: in it there has to be identity of essence and existence.
God is pure act, pure actuality totally unfolded, and every other substance participates of His being. Therefore, matter and form ‘are’, are ‘being’, because they participate in the being of the pure act, of the primal cause. In this way Thomas succeeds in giving theoretical basis to the dogma of creatio ex nihilo sui et subiecti and the hierarchy of entia.
For the sake of clarity, let us now try to quickly and schematically go over this ontological structure in reverse. It consists of three orders:
The first cause of creation is God, who is the simplest primal act. In him essence and being are identical, and every other thing participates in His being. This identity is, for Thomas, the meaning of the biblical sentence ‘egò eimi ho òn’ (‘I am what is’). God is the first ens (primo ente quod est esse tantum) and first cause (et hoc est causa prima, quae Deus est). God, however, is different from the universal concept of Being, because God implies the exclusion of any addition: he possesses all the perfections that are present in any other genera, and, in fact, possesses them in a more excellent way because they are one.
As one proceeds further away from the first being, the degree of perfection decreases.
These are what the philosophers call ‘intelligentiae’ and biblical revelation ‘angels’. In them there is a composition of ‘form’ and ‘act of being’, and their form is also their substance. The form of these substances is in potential in relation to being: in order to exist (actualise) they need to participate in the being the primal cause. They are, of course, immaterial and various, but totally imperceptible by human beings. In fact, it is possible to define the identity between their form and their essence, but nothing about their accidents.
There is a median element here: the human soul is part of separate substances, but at the same time, it is embodied in matter. It therefore acquires its individuality in regard to matter and retains it even when it is freed of its relationship. The human being is halfway between simple and composite substances.
These are the entities that can be perceptible by the human beings in re. They are made of ‘matter’, ‘form’ and ‘act of being’, and are therefore material, potential and multiple. Their substance is the composite of matter and form, and the accidents that are joined to them do not have substance in themselves but acquire it in merging with the substance to which they belong.
Thomas Aquinas completes the project of establishing a strong theoretical philosophical basis for Christian revelation by intelligently acquiring and treasuring Greek philosophy, particularly Aristotelian philosophy. For this reason, Thomas’ theoretical framework will be the reference for the neo-scholastic philosophical movement, and still represents a high point of human thought today.
 In English there is no adequate term for translating the Latin term ‘ens, entis’ (something that is). The translator of the work himself used the term “Being” which, however, can also mean ‘Something that is’ and ‘the thing that makes everything that is, is’ or ‘the common quality that unites everything that is, or that exists’. It has therefore been decided to distinguish the two terms in the following manner: “Ens” as ‘something that is’ and “Being” as ‘the thing that makes everything that is, is’. In this article, when these terms are found in Thomas’ quotes, the term “Ens” will be in squared brackets, next to the term “Being”– whenever the former is the translation of the latter – whereas Being is to be understood as ‘the thing that makes everything that is, is’. In this article I will simply use the terms “Ens” and “Being”, whereas “Entia” for the plural form of “Ens”.
 St. Thomas Aquinas, On Being and Essence [De ente et essentia], translated by Armand Maurer, Chapter II.
 Ivi, Chapter V.