The Ways to Wisdom
Aquinas Day By Day


Aquinas’s topic:  Logic of concepts: types of definitions

Scripture:  “Blessed fare you, Simon son of Johan.  For flesh and blood has not revealed this to you, but my heavenly Father.  And so I say to you, you are Peter, and upon this rock I will build my Church, and the gates of the netherworld shall not prevail against it.  I will give you the keys to the kingdom of heaven.  Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.” Mat 16: 15-19

Aquinas’s text:  Sententia libri de anima, Bk. 1, lec. 2, n. 26-8

Definitions of a species in terms of its proximate genus and specific difference are the ideal, so might be called logical definitions.  But this ideal is seldom achieved.  So in line with Aristotle’s understanding of definition, Br. Thomas made use of the four causes to develop what might be called causal definitions.  He also recognized that properties can be used to define things, as in Aristotle’s “political animal” as a definition of humans.  Here early in his commentary on Aristotle’s On the Soul, he compares definitions based on whether the use matter or form or both, and he compares the use made of these definitions in the physics, mathematics, and metaphysics.

26] So we have three kinds of definition. The first states the species and the definition of the species, but is purely formal, as when house is defined as “a shelter to keep out wind, rain, and heat.” The second kind states the matter, as when a house is said to be “a shelter made of stones and beams and wood.”  The third kind states, that is, includes in the definition “both,” namely matter and form, saying that a house “this kind of shelter, built of these kinds of materials, for this  purpose, namely, to keep out the wind, etc. Therefore, [Aristotle] says that “another” definition has three elements: namely, “in these,” that is, in beams and stones which are its material jparts; its “shape,” that is, its form; and “for these reasons,” that is, to keep out the wind. So he includes matter when he says “in these,” form when he says “shape,” and the final cause when he says “for these reasons.”  All three of these are required for a perfect definition.

27] To the question which of these types of definition is used by the natural scientist, I reply:  The purely formal definition is not used in natural science but in logic.  The definition that includes matter but omits form is used by no one but the natural scientist, because no one has to consider matter but the natural scientist.  But the definition that includes both, namely, matter and form, is more appropriate to the natural scientist.  Now these two kinds of definitions pertain to natural science, but one is imperfect, namely, the one that states only matter, while the other is perfect, namely, the one that includes the form also. For only the physical scientist studies the inseparable attributes of matter.

28] Since there are various ways of considering the attributes of matter, Aristotle now proceeds to show who they are and how they consider matter, dividing them into three classes. One kind differs from the natural scientist in its principle, although it considers attributes as existing in matter; such as the craftsman, who considers from in matter, but differs from the scientist because his principle is art, while the principle of the natural scientist is nature.  A second kind considers forms things that exist in sensible matter but does not include sensible matter in its definitions; such as curved, straight, and such.  Although they have existence in matter and are counted among things not separate from matter in their existence, nevertheless the mathematician about sensible matter for them.  The reason is that since there are some things that are sensible owing to their quality, but quality presupposes quantity, it follows that the mathematician is concerned only with what belongs to quantity as such, not what is limited to this or that matter. Finally, the third kind considers things whose existence is either completely independent of matter or whose existence can be without matter.  And this is the first philosopher.

26] Et sic habemus tres definitiones, quia una assignat speciem et speciei rationem, et est formalis tantum, sicut si definiatur domus quod sit operimentum prohibens a ventis et imbribus et caumatibus. Alia autem assignat materiam, sicut si dicatur quod domus est operimentum quoddam ex lapidibus, lateribus et lignis. Alia vero assignat idest in definitione ponit utrumque, materiam scilicet et formam; dicens, quod domus est operimentum tale constans ex talibus, et propter talia, scilicet ut prohibeat ventos etc. et ideo dicit quod alia definitio scilicet, tria ponit in his scilicet lignis lapidibus quae sunt ex parte materiae speciem idest formam propter ista scilicet ut prohibeat ventos. Et sic complectitur materiam cum dicit in his et formam cum dicit speciem et causam finalem cum dicit propter ista: quae tria requiruntur ad perfectam definitionem.

27] Sed si quaeratur quae istarum definitionum sit naturalis, et quae non: dicendum, quod illa, quae considerat formam tantum, non est naturalis, sed logica. Illa autem, quae est circa materiam, ignorat autem formam, nullius est nisi naturalis. Nullus enim habet considerare materiam nisi naturalis. Nihilominus tamen illa quae ex utrisque est, scilicet ex materia et forma, est magis naturalis. Et duae harum definitionum pertinent ad naturalem: sed una est imperfecta, scilicet illa quae ponit materiam tantum: alia vero perfecta, scilicet illa quae est ex utrisque. Non enim est aliquis qui consideret passiones materiae non separabiles, nisi physicus.

28] Sed quia sunt aliqui, qui aliter considerant passiones materiae, ideo ostendit qui sint, et qualiter considerent: et dicit quod sunt tres. Unum genus est quod differt a naturali quantum ad principium, licet consideret passiones prout sunt in materia; sicut artifex, qui considerat formam in materia, sed differunt, quia huiusmodi principium est ars, physici vero principium est natura. Aliud genus est quod quidem considerat ea quae habent esse in materia sensibili, sed non recipit in definitione materiam sensibilem; sicut curvum, rectum et huiusmodi, licet habeant esse in materia, et sint de numero non separabilium, quantum ad esse, tamen mathematicus non determinat sibi materiam sensibilem. Cuius ratio est, quia res aliquae sunt sensibiles per qualitatem, quantitates autem praeexistunt qualitatibus, unde mathematicus concernit solum id quod quantitatis est absolute, non determinans hanc vel illam materiam. Aliud genus est quod quidem considerat illa quorum esse vel non est in materia omnino, vel quorum esse potest esse sine materia; et hic est philosophus primus.

[Introductions and translations © R.E. Houser]

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