[4.9.4] St Thomas Aquinas on the Classification of Sciences

St Thomas Aquinas (1225 – 1274 AD), in his book On The Divisions and Methods of the Sciences: Questions V and VI of his Commentary on the De Trinitate of Boethius writes about the division of sciences (see also [1.3.10])

  • Sciences can be speculative or practical, depending on their relation to truth. Speculative sciences contemplate, while practical sciences apply truth.
  • We can analyze the object of study of the speculative sciences based on their dependence on the existence and understanding of matter and motion.
  • Based on this analysis, the complete list of speculative sciences is physics, mathematics, and metaphysics.
  • Ethics, economics and politics are practical sciences.

The following OntoUML diagram presents Aquinas’s division of sciences:

Aquinas on the division of sciences
ClassDescriptionRelations
Science“There are [..] two distinct classes of science: speculative science and practical science. Speculative sciences are those that contemplate truth whereas practical sciences are those that apply truth for some practical purpose. The sciences are then further distinguished through differentiating their various subject-matters.”studies SubjectMatter
Understanding“Working within the Aristotelian tradition, Thomas holds that something is understood when it is separated from matter and is necessary to thing in some respect. For instance, when we understand the nature of a tree, what we understand is not primarily the matter that goes to constitute the tree in question, but what it is to be a tree, or the structuring principle of the matter that so organizes it and specifies it as a tree rather than a plant. Furthermore, assuming our understanding is correct, when we understand a thing to be a tree, we do not understand it to be a dog, or a horse, or a cat. Thus, in our understanding of a tree, we understand that which is necessary for the tree to be a tree, and not of anything that is not a tree. Hence, our understanding of a thing is separated from its matter and is necessary to it in some respect. Now, what is in motion is not necessary, since what is in motion can change. Thus, the degree to which we have understood something is conditional upon the degree to which it is separated from matter and motion. It follows then that speculative objects, the subject-matter of the speculative-sciences, insofar as they are what are understood, will be separated from matter and motion to some degree. Any distinctions that obtain amongst speculative objects will in turn signify distinctions amongst the sciences that consider those objects; and we can find distinctions amongst speculative objects based upon their disposition towards matter and motion.”relates between Science and SubjectMatter
PracticalSciencepractical sciences are those that apply truth for some practical purpose.”subkind of Science; applies Truth
Ethics; Economics PoliticsEthics, economics and politics are practical sciences.subkind of PracticalScience
SpeculativeScienceSpeculative sciences are those that contemplate truth […]
There are three divisions that can apply to speculative objects, thereby permitting us to differentiate the sciences [to physics, mathematics and metaphysics…]
Thomas takes this division of the speculative sciences as exhaustive. For Thomas, there could be no fourth speculative science; the reason for this is that the subject-matter of such a science would have to be those things that depend on matter and motion for their being understood but not for their being, for all other combinations have been exhausted. Now, if a thing depends on matter and motion for its being understood but not for its being, then matter and motion would be put into its definition, which defines a thing as it exists. But if a thing’s existence is so defined as to include matter and motion, then it follows that it depends on matter and motion for its being; for it cannot be understood to be without matter and motion. Hence, all things that include matter and motion in their definitions are dependent on matter and motion for their being, but not all things that depend on matter and motion for their being depend on matter and motion for their being understood. There could be no fourth speculative science since there is no fourth class of speculative objects depending on matter and motion for their being understood but not for their being. Thomas thus sees this threefold division of the speculative sciences as an exhaustive one.”
subkind of Science; contemplates Truth
Physics“(i) physical science considers those things that depend on matter and motion both for their being and for their being understood”subkind of SpeculativeScience; studies ObjectOfStudy OfPhysics
Mathematics “(ii) mathematics considers those things that depend on matter and motion for their being but not for their being understood”subkind of SpeculativeScience; studies ObjectOfStudy OfMathematics
Metaphysics“(iii) metaphysics or theology deals with those things that depend on matter and motion neither for their being nor for their being understood.”subkind of SpeculativeScience; studies ObjectOfStudy OfMetaphysics
SubjectMatter“In order to ascertain the subject-matter of any particular science, Thomas distinguishes between the different intellectual operations that we use when engaged in some particular scientific endeavor. Broadly speaking, these fall into two categories: the speculative and the practical. Concerning some sciences, the intellect is merely speculative  by contemplating the truth of some particular subject-matter; while concerning other sciences, the intellect is practical, by  ascertaining the truth and seeking to apply. There are thus correspondingly two distinct classes of science: speculative science and practical science. Speculative sciences are those that contemplate truth whereas practical sciences are those that apply truth for some practical purpose. The sciences are then further distinguished through differentiating their various subject-matters.”
ObjectOfStudy OfPhysics“(i) there is a class of speculative objects [object of study of physics] that are dependent on matter and motion both for their being and for their being understood, for instance, human beings cannot be without matter, and they cannot be understood without their constituent matter (flesh and bones)”subkind of SubjectMatter
ObjectOfStudy OfMathematics“(ii) there is a class of speculative objects [object of study of mathemathics] that depend on matter and motion for their being, but not for their being understood, for instance, we can understand lines, numbers, and points without thereby understanding the matter in which they are found, yet such things cannot be without matter”subkind of SubjectMatter
ObjectOfStudy OfMetaphysics” (iii) there is a class of speculative objects [object of study of metaphysics] that depend on matter and motion neither for their being nor for their being understood.
[…] As it is a purely rational science, not dependent on or presupposing the truths of revelation, metaphysics will be a study of the neutrally immaterial aspects of things, that is, a study of those modes of being that apply to all beings, whether they are material or immaterial. 
Thomas does not adopt the Aristotelian phrase (being qua being) as the subject-matter of metaphysics, he offers his own term. According to Thomas, ens commune (common being) is the proper subject-matter of metaphysics. Through an investigation of ens commune, an investigation into the aspects of being common to all beings, the metaphysician may indeed come to a knowledge of the causes of being and might thereby be led to the affirmation of divine being, but this is only at the end of the metaphysical inquiry, not at the beginning.”
subkind of SubjectMatter

Sources

  • All citations from:  Kerr, Gaven : “Aquinas: Metaphysics”Internet Encyclopedia of Philosophy
  • McInerny, Ralph and John O’Callaghan, “Saint Thomas Aquinas”The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.)
  • The Cambridge Companion to Aquinas, CAMBRIDGE UNIVERSITY PRESS, Edited by  Norman Kretzmann and Eleonore Stump, 2010

First published: 01/10/2020

[4.9.3] St Thomas Aquinas on Essence and Existence in Immaterial Substances

St Thomas Aquinas (1225 – 1274 AD), in his work On Being and Essence analyzes substances using concepts like matter, form, hylomorphism, potentiality, actuality (developed by Aristotle, see [1.3.5]), essence, and existence (developed by Avicenna, see [3.3.1]).

  • Substances are enduring primary existents, which can be material or immaterial, (see also [4.9.2]).
  • Substances are a composition of essence (which makes the substance what is) and existence (which is the act of being “esse“).
  • The essence of immaterial substances is their form.
  • For immaterial substances, the form is the potentiality, and existence is the actuality.
  • The essence (quiddity) of God, the first existent – is its existence. God gives existence to all the other substances, be it material or immaterial.

The following OntoUML diagram presents Aquinas’s model of immaterial substances:

Aquinas on essence and existence in immaterial substances

ClassDescriptionRelations
SubstanceAccording to Aquinas substances are what are primarily said to exist. They are enduring things like trees, horses, humans.
“Some substances are simple [immaterial], others are composite [material]”.

MaterialSubstanceMaterial substances [composite substances] are like: Socrates, a horse, a tree, a stone etc.
“Some substances are simple, others are composite, and both sorts have their essence, but the simple ones in a more genuine and excellent way, just as they have a more excellent way of being. For they are the cause of the composite ones; at least this is true of the first, simple substance, which is God.
However, since the essences of these substances are quite hidden from us, we should begin with the essences of composite substances, so that our discussion may more suitably proceed from the easier subjects.”
subkind of Substance
ImmaterialSubstanceImmaterial substances [simple substances]: are like: soul, intelligences [i.e., angels], and the first cause [i.e., God].
They have their essence “in a more genuine and excellent way, just as they have a more excellent way of being. For they are the cause of the composite ones; at least this is true of the first, simple substance, which is God. […]
Therefore, in the soul or in the intelligences there is no composition of matter and form in any way, so that they could be understood to have matter in the same sense in which corporeal substances do. But there is the composition of form and existence [esse] in them.”
subkind of Substance
ImmaterialSubstance (OtherThanGod)Immaterial substance other than God are like soul, intelligences [i.e., angels].
“It is clear that forms are not actually intelligible, unless they are separated from matter and its [individuating] conditions, and they are not rendered actually intelligible, except by the power of an intelligent substance, insofar as it receives them and works on them. Therefore, an intelligent substance has to be immune from matter in every way, so that it neither has matter as its part, nor does it exist as a form impressed in matter, as is the case with material forms.”
subkind of ImmaterialSubstance
Essence“For on the basis of what has been said so far it is clear that the essence of a thing is what its definition signifies.”exclusive part of ImmaterialSubstance
Essence (ofImmaterial Substance OtherThanGod)“The difference, therefore, between the essence of a simple substance and that of a composite substance is that the essence of a composite substance is not only the form, but it comprises both form and matter, whereas the essence of a simple substance [essence of Immaterial substance] is its form only. […]
since the essence of a simple thing is not received in matter, in their case there cannot be this kind of multiplication; therefore, in the case of these substances, there cannot be several individuals in the same species, but there are as many species as there are individuals, as Avicenna expressly claims. […]
the essence of a simple thing, which is its form, can only be signified as a whole, for there is nothing there apart from the form, so there is nothing that could receive the form. Therefore, no matter how we consider the essence of a simple substance, it is predicated of the simple substance. This is why Avicenna says that the quiddity of a simple substance is the simple substance itself, for there is nothing else [in the simple substance] to receive it.”
subkind of Essence; exclusive part of ImmaterialSubstance
Definition“the definition of things of nature contains not only form, but matter as well; otherwise natural definitions would not differ from mathematical definitions.”
The definitions are in the mind.
signifies Essence
Form“So, those forms that are the closest to the first principle are forms subsisting without matter, for the whole genus of forms does not require matter, as has been said, and forms of this sort are the intelligences; so, the essences or quiddities of these substances do not have to be other than their form itself. […]characterizes Essence (ofImmaterial Substance OtherThanGod)
Existence“Whatever is not included in the understanding of an essence or quiddity is coming to it from outside, entering into composition with the essence; for no essence can be understood without its parts. But every essence can be understood without even thinking about its existence, for I can understand what a man or a phoenix is, and not know whether it actually exists in the nature of things. Therefore, it is clear that existence is distinct from essence […]”characterizes Essence (ofImmaterial Substance OtherThanGod)
Potentialty“Therefore, the quiddity or form that is the intelligence has to be in potentiality with respect to the existence it receives from God […]”relates Form with Essence (ofImmaterial Substance OtherThanGod)
Actuality“Therefore, the quiddity or form that is the intelligence has to be in potentiality with respect to the existence it receives from God, while the existence received is its actuality.relates Existence with Essence (ofImmaterial Substance OtherThanGod)
GodGod, the first cause.
“Now, everything that a thing has is either caused in it by its own principles, as the ability to laugh in man, or it comes to the thing from an external source, as the light in the air is coming from the sun. But the existence of a thing cannot be caused by its form or quiddity itself (I mean, as by an efficient cause), for then a thing would be its own cause, and would bring itself into existence, which is impossible. Therefore, all such things, namely, those that have their existence as something distinct from their nature, have to have their existence from something else. However, since everything that is through something else [per aliud] is reduced to what is through itself [per se] as its first cause, there has to be something that is the cause of existence for everything, since it is existence only. For otherwise the series of causes would go to infinity, since every thing that is not existence only has a cause for its existence, as has been said. It is clear, therefore, that an intelligence is both form and existence, and that it has its existence from the first being that is existence only; and this is the first cause, which is God.”
subkind of ImmaterialSubstance; causes Existence
God’sExistenceGod’s existence is his essence (quiddity).subkind of Existence; characterizes God

Sources

  • All citations from:  Thomas Aquinas, “On Being and Essence”, in Medieval Philosophy: Essential Readings with Commentary, Edited by Gyula Klima with Fritz Allhoff and Anand Jayprakash Vaidya, Blackwell Publishing Ltd, 2007
  • Kerr, Gaven : “Aquinas: Metaphysics”, Internet Encyclopedia of Philosophy
  • McInerny, Ralph and John O’Callaghan, “Saint Thomas Aquinas”The Stanford Encyclopedia of Philosophy (Summer 2018 Edition), Edward N. Zalta (ed.)
  • The Cambridge Companion to Aquinas, CAMBRIDGE UNIVERSITY PRESS, Edited by  Norman Kretzmann and Eleonore Stump, 2010
  • Joshua P. Hochschild, “Form, Essence, Soul: Distinguishing Principles of Thomistic Metaphysics”

First published: 24/09/2020