[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

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