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Aquinas on Varieties of Necessity in the Created World
Published online by Cambridge University Press: 18 September 2025
Abstract
This essay examines Aquinas’s views on necessity in the created world. Although Aquinas holds that all created being is contingent upon God’s free act of creation, he nevertheless maintains that there are aspects of the created world that cannot be otherwise. This raises difficult questions about how such necessities arise in a contingent world and how they relate to God’s power. Aquinas’s analysis is complicated by his view that “necessity” is said in many ways. In various contexts, he distinguishes between absolute, natural, material, conditional, intrinsic, and extrinsic necessity. The essay offers a roadmap through these diverse kinds of created necessity, clarifying their sources and interrelations. It also considers the diverse ways Aquinas deploys the term “absolute necessity” in different contexts and explores how created necessities relate to God’s power.
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- © The Author(s), 2025. Published by Cambridge University Press on behalf of Provincial Council of the English Province of the Order of Preachers.
References
1 See for example ST Ia.82.1 co.: ‘necesse enim est, quod non potest non esse’; In. III Sent. 16.1.2 co.: ‘necessarium idem est, quod impossible non esse’.
2 See especially ScG II.30.
3 ST Ia.82.1 co.
4 For other studies that discuss some of Aquinas’s various uses of ‘necessity’, see Shane Drefcinski, ‘Necessary Truths and St. Thomas Aquinas’, Definition of ‘Law’, Canadian Journal of Law and Jurisprudence, 37:2 (2024), 601–617 and J.J. Macintosh, ‘Aquinas on Necessity’, American Catholic Philosophical Quarterly, 72: 3 (1998), 371–403. The present study goes beyond this literature by focusing on Aquinas’s different uses of ‘absolute’ necessity and how his conception of ontological absolute necessities in the created world relate to his understanding of absolute logical necessities.
5 ScG II.30: ‘… absoluta necessitas in rebus creatis non est per ordinem ad primum principium quod per se necesse est esse, scilicet Deum, sed per ordinem ad alias causas, quae non sunt per se necesse-esse.’
6 ScG II.30: ‘Si vero comparentur ad principia proxima, inveniuntur necessitatem habere absolutam.’
7 Ibid.
8 In V Meta. lec. 1: ‘[H]oc vero nomen causa, importat influxum quemdam ad esse causati.’
9 De pot. 5.1: ‘Effectum enim a sua causa dependere oportet. Hoc enim est de ratione effectus et causae…’
10 ScG II.30: ‘Diversimode autem ex diversis causis necessitas sumitur in rebus creatis.’
11 ST Ia 19.3.
12 In I Peri Hermeneias lec. 15: ‘… illud quod non est absolute necessarium, fit necessarium ex suppositione eiusdem, quia necesse est esse quando est …’
13 ST Ia 19.3. For the same reason, if it is supposed that something that is not necessary in itself, i.e., in virtue of its terms, happened in the past, it too becomes necessary on this supposition. See De pot. 1.3 ad 9.
14 In I Peri Herm. lec 15: ‘… quod omne quod est necesse est esse quando est, et omne quod non est necesse est non esse quando non est. Et haec necessitas fundatur super hoc principium: impossibile est simul esse et non esse: si enim aliquid est, impossibile est illud simul non esse; ergo necesse est tunc illud esse.’
15 Ibid. This principle applies to propositions of all tenses. However, Aquinas thinks that it is not possible to form necessarily true conditional propositions about the future by ‘supposing the same’, e.g., ‘Supposing Socrates will sit, then necessarily Socrates will be sitting while he will be sitting’ since future tense propositions cannot have truth-values in his view. For discussion see Stephen Brock, ‘G.E.M. Anscombe and Thomas Aquinas on Necessity and Contradiction in Temporal Events’, in Analytical Thomism, edited by C. Paterson and M.S. Pugh (Farnham, U.K.: Ashgate, 2006), pp. 283–302. For an alternative interpretation of Aquinas’s views on the necessity of the past and present, see Robert Pasnau, ‘Medieval Modal Spaces’, Aristotelian Society Supplementary Volume, 94:1 (2020), 225–254.
16 De ver. 23.4 ad 1: ‘Absolute quidem dicitur aliquid necessarium propter necessariam habitudinem ad invicem terminorum qui in aliqua propositione ponuntur; sicut hominem esse animal, vel omne totum esse maius sua parte, aut aliqua huiusmodi.’
17 ST Ia 19.3 co. and De ver. 23.4 ad 1. He likewise claims that true and false and possible and impossible predicated absolutely (i.e., in virtue of the terms within a proposition itself) and given a supposition (In I De Caelo lec. 26).
18 ST Ia 82.1 co. and In V Meta. l. 6.
19 Ibid.
20 In V Meta. lec. 6: ‘… quod necessarium etiam dicimus sic se habere, quod non contingit aliter se habere: et hoc est necessarium absolute’.
21 In IV Sent. 7.1.1.2.: ‘Una est necessitas absoluta, sicut necessarium est Deum esse, vel triangulum habere tres angulos.’
22 In V Meta. lec. 6.
23 ST Ia 82.1: ‘Quod quidem convenit alicui uno modo ex principio intrinseco, sive materiali, sicut cum dicimus quod omne compositum ex contrariis necesse est corrumpi; sive formali, sicut cum dicimus quod necesse est triangulum habere tres angulos aequales duobus rectis. Et haec est necessitas naturalis et absoluta.’
24 ST III 14.2: ‘Alia autem est necessitas naturalis, quae consequitur principia naturalia, puta formam, sicut necessarium est ignem calefacere; vel materiam, sicut necessarium est corpus ex contrariis compositum dissolvi.’
25 See section two of Kment, Boris, ‘Varieties of Modality’, The Stanford Encyclopedia of Philosophy (Spring 2021 Edition), Edward N. Zalta (ed.), URL = <https://plato.stanford.edu/archives/spr2021/entries/modality-varieties/>.
26 In ST Ia 82.1, Aquinas claims that the necessity that follows from matter and form is ‘natural and absolute’. Robert Pasnau has pointed out that there is an ambiguity in whether the term ‘natural necessity’ should be taken to refer solely to material necessity. See his commentary and translation of Thomas Aquinas, Treatise on Human Nature (Summa theologiae 1a 75-89) (Indianapolis: Hackett, 2002), p. 310. Pasnau’s diagram on this page also reflects this ambiguity. Some commentators identify material necessity with natural necessity and formal necessity with absolute necessity. See Hester Gelber, It Could Have Been Otherwise: Contingency and Necessity in Dominican Theology at Oxford 1300-1350, (Leiden: Brill), 2004, pp. 115–116. Against this interpretation, the texts quoted above in notes 22 and 23 make clear that Aquinas thought that the necessities that follow from matter and form are together both absolute necessities and natural necessities.
27 In V Meta. lec. 6: ‘Necessarium autem secundum quid et non absolute est, cuius necessitas dependet ex causa extrinseca.’
28 ST Ia 82.1: ‘Ex agente autem hoc alicui convenit, sicut cum aliquis cogitur ab aliquo agente, ita quod non possit contrarium agere. Et haec vocatur necessitas coactionis. Haec igitur coactionis necessitas omnino repugnat voluntati’.
29 In V Meta. lec. 6
30 De ver. 17.3 and In IV Sent. 29.1.1.
31 ScG II.30.
32 De malo 2.3 ad 4.
33 In the replies of In II Sent. 29.1.1 and In IV Sent. 7.1.1.2, Aquinas distinguishes between two types of necessity from an end, namely, things that are conditions for an end to be achieved simpliciter and things that are necessary for an end to be easily achieved or achieved well. Food is necessary for life in the former way, and a ship is necessary to cross the sea in the latter way.
34 In Meta. V lec. 6. See also In II Sent. 29.1.1 and In IV Sent. 7.1.1.2.
35 In Meta. V lec. 6.
36 Ibid.
37 Ibid.: ‘Nam necessarium est, quod est impossibile aliter se habere’.
38 De ente, c. 4; In II Phys. lec. 15 and ScG II.29.
39 For Aristotle’s discussion of the two divisions, see Physics 199b 34–200b 9 and Metaphysics 1015a 20-1015b 15.
40 In II Phys. lec. 15: ‘… necessitas quae dependet ex causis prioribus, est necessitas absoluta … Quod autem habet necessitatem ab eo quod est posterius in esse, est necessarium ex conditione, vel supposition …’ See also De ente, c. 4; ScG II.29.
41 ST Iae 1.1 ad 1. Avicenna is the source for this view. For discussion of Aquinas’s views, see Gloria Frost, Aquinas on Efficient Causation and Causal Powers (Cambridge: Cambridge University Press, 2022) section 4.2.
42 De prin. nat. c. 3 (ed. Leon., vol. 43, 42): ‘… et hoc intendere nichil aliud erat quam habere naturalem inclinationem ad aliquid’.
43 ScG II.29: ‘Necessitas autem quae est a posteriori in esse licet sit prius natura, non est absoluta necessitas, sed conditionalis: ut, si hoc debeat fieri, necesse est hoc prius esse’.
44 De ente, c. 4.
45 In the prior chapter (ScG II, c. 29), he defines absolute necessities as those that follow from prior causes.
46 For discussion of Aquinas’s views see Frost, Aquinas on Efficient Causation and Causal Powers.
47 ST III 8.5.
48 ScG II.30.
49 ScG II. 30: ‘Sicut enim alia accidentia ex necessitate principiorum essentialium procedunt, ita et actio ex necessitate formae per quam agens est actu: sic enim agit ut actu est.’
50 On impediments to efficient causes, see ScG III.73.
51 ScG II.30.
52 Ibid.
53 Ibid.
54 ST III 14.2.
55 In II Phys. lec. 15: ‘Et similiter quod habet necessitatem ex causa efficiente, est necessarium absolute; sicut necessarium est esse alternationem noctis et diei propter motum solis.’
56 ScG II.30.
57 De pot. 5.8.
58 ScG II.30.
59 De ver. 9.1: ‘… nulla res, quantumcumque materialis, recipit aliquid secundum id quod est formale in ipsa, sed solum secundum id quod est materiale in ea … res corporales non recipiunt aliquam impressionem ex parte formae, sed ex parte materiae …’ For discussion, see Frost, Aquinas on Efficient Causation and Causal Powers, section 5.1.
60 In the first division, the necessities that follow from a being’s intrinsic principles are contrasted with relative (secundum quid) necessities. Though the agent cause is extrinsic to the patient, absolutely necessary effects in efficient causation are certainly not relative necessities. Though such necessities require the existence of both agent and patient, rather than just the existence of a single substance as formal or material necessities do, they differ from relative necessities because they obtain in every possible circumstance in which both the agent and patient exist. They do not require any other further condition beyond existence of the two, agent and patient substance. In that respect, they are unlike necessities that obtain only under certain extrinsic circumstances.
61 ScG II.28–30.
62 ScG II.30: ‘Nihil enim prohibet aliqua principia non ex necessitate produci, quibus tamen positis, de necessitate sequitur talis effectus: sicut mors animalis huius absolutam necessitatem habet propter hoc quod iam ex contrariis est compositum, quamvis ipsum ex contrariis componi non fuisset necessarium absolute.’
63 ST Ia 25.3: ‘… Deus dicatur omnipotens, quia potest omnia possibilia absolute…’
64 Ibid.: ‘Dicitur autem aliquid possibile vel impossibile absolute, ex habitudine terminorum, possibile quidem, quia praedicatum non repugnat subiecto, ut Socratem sedere; impossibile vero absolute, quia praedicatum repugnat subiecto, ut hominem esse asinum.’
65 ST Ia 97.1 co.
66 See, for instance, De pot. 5.5.
67 ScG II.25: ‘Ad remotionem cuiuslibet principii essentialis sequitur remotio ipsius rei. Si igitur Deus non potest facere rem simul esse et non esse, nec etiam potest facere quod rei desit aliquod suorum principiorum essentialium ipsa remanente: sicut quod homo non habeat animam’.
68 ScG II.25: ‘Cum principia quarundam scientiarum, ut logicae, geometriae et arithmeticae, sumantur ex solis principiis formalibus rerum, ex quibus essentia rei dependet, sequitur quod contraria horum principiorum Deus facere non possit: sicut quod genus non sit praedicabile de specie; vel quod lineae ductae a centro ad circumferentiam non sint aequales; aut quod triangulus rectilineus non habeat tres angulos aequales duobus rectis’.
69 Medieval Aristotelians debated this point and Aquinas followed Avicenna’s position, rather than Averroes’s view that essences of material creatures contained only formal features. On this debate, see Frost, ‘Medieval’, in The Routledge Handbook of Essence, edited by K. Koslicki and M. Raven, (New York, NY: Routledge, 2024), pp. 30–40.
70 Super De Trinitate 5.3: ‘… per se competit homini quod inueniatur in eo anima rationalis et corpus compositum ex quatuor elementis, unde sine his partibus homo intelligi non potest, set hec oportet poni in diffinitione eius …’
71 God only wills his own goodness as an end with necessity and no creatures are required to be willed by him in order to necessarily will his own goodness. See, for instance, ScG I.81.
72 ScG II.28–29.
73 ScG II, 28: ‘… non enim potest esse quod Deus aliquid se facturum disposuerit quod postmodum ipse non faciat; alias eius dispositio vel esset mutabilis vel infirma.
74 ScG II.29: ‘… Deus creaturae debitor non dicitur, sed suae dispositioni implendae.’
75 ScG II.29. See also De Pot. 3.16.
76 ST Ia 105.6.
77 ScG III.101: ‘… Deus facit quod consuetum est fieri operatione naturae, tamen absque principiis naturae operantibus: sicut cum aliquis a febre curabili per naturam, divina virtute curatur; et cum pluit sine operatione principiorum naturae.’
78 Ibid.