2.1. An individual substance of a rational nature

In the first article of ST I, q. 29, Aquinas takes up the definition of the “person” given by Boethius, “the person is an individual substance of a rational nature” (³) by excluding three orders of realities (^{6, p. 24}) incompatible with it: accidents,⁴ genera and species in the genus of substance and inanimate bodies, plants and animals.

Reformulating Aristotle, Aquinas indicates that the word “substance” can have two meanings: either it expresses the quiddity of the thing, its definition: ousia, essentia or nature, which is called second substance. Or it expresses the subject itself, the individual henchman subsisting in the genus substance, which is designated by the first substance. By affirming that the person is a substance “taken in a general sense which dominates the subdivisions (first substance and second substance)” (³), he excludes accidents because the substance is “individuated by its own principles and not by any other external thing”, as he asserted in article 2 of question 9 in his treatise De potentia⁷. Analyzing the subdivisions of substance (⁸), Aquinas affirms that the person is the primary individual substance which bears within itself three characteristics (^{6, p. 29-35}) reported to subject ³: subsistentia (exists by itself and not in another), res naturae (subject of a common nature) and hypostasis (subject of accidents). Therefore, the person as primary substance designates an individuation of a being that exists in itself and not in another or through another. It “possesses its own being in a complete way, in itself and by itself, exercising on its own account the act of existing” (^{4, p. 31}).

As for the term “nature”, Aquinas considers it as an intrinsic principle of all movement. Now, since the principle can be formal or material, and since, as Boethius says, “nature is what informs each thing by endowing it with its specific difference”, the word “nature” can be understood as a formal principle which gives form or essence to first substance as its formal cause as expressed by Christian Erk (⁹).

As far as “rationality” is concerned, it implies will and freedom. It is an inclination, this “intellectual appetite” which means that beings endowed with intelligence not only tend towards an end, but “they have the capacity to carry themselves, by their will, towards the end” (^{4, p. 132}) apprehended by intelligence while controlling their actions. How does this substance manifest in humans?

2.2. The substantial unity of soul and body

To define the substance of man, Aquinas begins with the foundation: according to ST I, q. 75, a. 1, the first principle of life (³) is the soul as a subsisting reality as Aquinas defined it in the first question of his treatise (De anima¹⁰). Fabrizio Amerini (^{11, p. 50-51}) explains it this way: a) Since the operation of intellect is immaterial, this supposes that the substance, the soul, endowed with such power is immaterial. b) Since through intelligence the soul can know the nature of all bodies, this assumes that it possesses nothing in itself of their nature according to what Aquinas explains in ST I, q. 75 and in the question 14 of De anima (³,¹⁰). Therefore, the soul is a subsisting reality through its immaterial intellectual operation in which the body does not intrinsically participate. Can we say that the person identifies with the soul?

Thomas Aquinas is not in favor of this opinion held by other authors (^{6, p. 88-97}). For him, the subsistence of the individual substance of a rational nature is perfect only in God, since in Him there is no division. Whereas in man as a bodily creature, the soul is not confused with the person. The body is “necessary for there to be a human person” (^{6, p. 99-100}). Now, how to understand the soul as a subsisting reality if the body is necessary for there to be a person?

Emmanuel Housset gives an answer to this question by stating that:

The strength of Saint Thomas here is to question the human person as a whole and not from one of its elements […]. It is by starting from the person as a whole that it is possible to elucidate his own form of existence without making it a simple being endowed with a substrate (^{5, p. 158-159}).

In fact, Aquinas did not invent a new aspect of the person, but he approached it and apprehended it differently by specifying that the person is not only a being endowed with a substrate, is not only a body animated by a soul but that the very essence of the person is this soul-body compound. The soul is the substantial form of the body and the union of the two is not an accidental but a substantial union for three reasons:

Thus, the substance of the person resides in the union of the soul with the body which “could not be accidental. […] The person therefore ceases to be considered simply as a spirit which incarnates itself in a particular matter” (^{5, p. 156}). Substantial union is an ontological necessity which, according to Elio Sgreccia, establishes the dignity of the human person from conception. However, this foundation comes up against the problem of the mediate animation of the embryo.

3.1. Formal discontinuity

Thomas Aquinas is among those who support mediate animation with a succession of substantial forms in the embryo. Based on the biological conceptions of Aristotle and Albertus Magnus (Saint Albert the Great), Aquinas explains -in article 9 of question 3 of De potentia- that the process of generation is a series of corruption in four phases (⁷):

It is clear that Aquinas cannot accept immediate animation ¹⁸. This theory of stages, or graduality according to Elio Sgreccia (^{2, p. 460-461}), is taken up by certain contemporary authors such as Claudio Antonio Testi (^{19, p. 8}), Aline Lizotte and Ernst Haeckel cited by Pascal Ide (^{20, p. 30-31}). However, from a metaphysical point of view, such a process of generations, corruptions and succession of souls poses the problem of the appearance of new species (^{11, p. 99-101}) and the embryonic identity and unity of the subject.

3.2. Subject unity

To be able to solve this dilemma, Thomas Aquinas offers two solutions.

4.1. Abortion

Based on the question of mediate rational animation, we could be in favor of abortion before this period because we do not consider the embryo as a human being. It seems that Aquinas goes in this direction by limiting himself to two explanations. On the theological level, he affirms that before rational animation, the embryos are excluded from the final resurrection (²¹), from the protection of the guardian angels (³,¹²) and from the possibility of sanctification (²³). Legally, although abortion is considered a grave sin, Aquinas considers it less than a homicide (¹¹, ^{21, p. 212-213, 218}; ²⁴).

Even with mediate animation, modern thinkers like Fabrizio Amerini (^{11, p. 211-212}) and Bruno Saintôt (^{25, p. 19}) consider abortion as an act against nature because embryos are not the end of a generative process; they are not created to remain “embryos” but to generate a human being. Others like Gianfranco Basti (^{26, pp. 356 ss.}), Noël Simard (²⁷), Tommaso Scandroglio (^{28, p. 3}) and John Meyer (^{29, p. 18-19}) -to reject abortion- consider that in the light of scientific and genetic discoveries, it is possible to affirm immediate animation. They consider that at the time of the fusion of the male and female gametes during fertilization, a new cell emerges carrying in it all the information (the 46 chromosomes) necessary to generate a human being.

4.2. Rationality

What about rationality in the embryo? Maurizio Mori categorically rejects it. For him, there is no rationality without the presence of an adequately formed cerebral cortex (^{30, p. 71-72}) and some neurological activity (^{29, p. 24-25}). Jacques Maritain agrees with this idea by identifying the presence of the soul with the development of the nervous system and the advanced sensory-motor psyche (^{31, p. 115}).

However, Aquinas in ST I, q. 76, a. 8, resp. affirms that the rational soul is not localized in a determined part of the body but, as a substantial form, it is found in the whole body as in each of its parts (³). To identify the soul with the capacity to develop an operation, shows the misunderstanding of two Thomasian principles explained in ST I q. 77, a. 6, resp. and in his work Quodlibets chap. ix, q. 2, a. 3, ad. 2. These are the principles of the operation and the powers of the soul which are accidents and which do not enter into the constitution of the substance of the individual (³,³²).

In addition, in the transition from potency to act in the embryo, we must distinguish between the first act and the second act according to Aristotelian metaphysics. In De anima q. 12, arg. 14 and in book ix of Sententia libri Metaphysica, Thomas Aquinas affirms that, for there to be a human being in act, it suffices that there is the rational soul; this is considered as the first act. The exercise of operations is considered as a secondary act (¹⁰,³³). Saying that the embryo is a potential person (^{11, p. 204}) is to affirm that the passage from the embryo to the man, from potency to act, does not mean the passage from non-exercise to the exercise of rationality. The latter is not the fact of having the possibility of possessing the capacity to reason, nor the possession of the capacity to reason in action, but the possession of the capacity to be able to reason. In ST I, q. 77, a. 1, Aquinas affirms that being a man does not depend on the use but on the possession of the capacity to be able to reason (³) realized either in a more or less perfect way, or never ²³ according to ST III, q. 68, a. 12. In this sense, Christian Erk affirms that a “person can only act rationally because he was a rational being from the start. […] There is no potential person but people with a potential […] to become the rational beings that they already are” (^{9, p. 108}) while Meyer does not seem to agree with this approach (^{29, p. 27-31}).

4.3. Twinning

Biologically, twinning can take place after fertilization and during the first two weeks of embryonic development. According to Pascal Ide and Angelo Serra (^{20, p. 165}; ^{34, p. 593}), it could be the result of a genetic error, but It can also considered “an exception” (^{20, p. 164}). The possibility of such an event also comes down to the potentiality of totipotent cells, the blastomeres, to become an independent being by separating themselves “from the extracellular material that holds them together” (^{20, p. 165}).

From the metaphysical and ontological point of view, the problem arises at the level of animation. For those who support mediate animation, the argument of the preservation of embryonic individuality and the impossibility of the division of the rational soul ¹⁴ is advanced in chapter 86 of SCG II. If the person is an “individual substance of a rational nature”, the animated embryo is an individual one, and undivided by itself. Twinning is only possible before the rational animation of the embryo. Pascale Ide (^{20, p. 21}), relying on Xavier Thévenot, and Bertrand de Margerie (^{35, p. 129}) are supporters of this concept.

Those who support immediate animation admit the individuality of the embryo from fertilization, even if twinning occurs. For them, the soul is present in the whole as in each part of the body as affirmed by Thomas Aquinas (¹⁰). During twinning, they prefer to speak of a phenomenon of loss or separation and not of division- of the embryonic material which is made up of totipotent cells. According to Jason Eberl, shortly after the separation of the totipotent cells, the latter are immediately informed by the rational soul (^{36, p. 39}). For Pascal Ide, “grasping the appearance of the twin requires reasoning no longer in metaphysics but in philosophy of nature”. For him, “the living body cannot be devoid of soul” (^{20, p. 167}). Monozygotic twins are ontologically identical. The only difference lies in how they appear (^{20, p. 169}).

It is difficult to decide between these two currents which put forward well-founded but also questionable arguments.