The Arguments of Aquinas

A Philosophical View

J.J. MacIntosh

• Publisher: Routledge
• Year: 2017
• Print ISBN: 9780367876098
• eBook ISBN: 9781351733779
• Edition: 1
• Topic: Philosophy
• Subtopic: Medieval & Renaissance Philosophy

The Arguments of Aquinas is intended for readers with philosophical interests, who may not be specialists in medieval philosophy. Some think that a medieval saint must be, as such, wrong, dated, and boring; others feel that a saint, any saint, must be right, relevant, and inspirational. Both groups are likely to misread Aquinas, if indeed they read him at all. The works of great philosophers are products of their times, but that does not lessen their value for us. We profit by reading the works of St Thomas in the same interested but critical way that we read the works of our contemporaries.

MacIntosh does not hesitated to compare Thomas's arguments with those of later philosophers as well as with those of his contemporaries and earlier philosophers. He chooses topics from a variety of still interesting problem areas: the existence and attributes of God, including God's foreknowledge and human free will, causality and the origin of the universe, time and necessity, human souls, angels, and the problem of evil. Additionally, the volume looks at his views on honesty and lying, and on human sexuality, on which he is, as ever, philosophically interesting whether or not we accept his conclusions.

Part I
Natural philosophy

1
Necessity and possibility¹

“Bucket! It’s not possible that Mr Tulkinghorn has been killed, and that you suspect me?”

“George,” returns Mr Bucket,… “it is certainly possible, because it’s the case.”

Dickens, Bleak House

St Thomas distinguishes a variety of notions of necessity, distinctions which have often been either overlooked or misunderstood by his detractors, and sometimes by his defenders. It is important to understand his different uses of necessity and possibility, in order not only to see what could count for and against his arguments, but even to understand what his arguments involve. In this chapter I consider some of the philosophically more important kinds of necessity Thomas distinguishes and uses. Unless we know what he understood in this or that context by necessity and related modal terms such as possibility and contingency, we cannot understand some of his points: Why is God’s changing the past impossible, for example, and why, though it is not our logical impossibility, is it at least as strong as what Thomas calls absolute necessity? Understanding Thomas’s sense of necessity in the third way is required if the argument is even to be discussed.

1.1. Aquinas’s modal vocabulary: syntax and interpretation

When we consider modal notions, we need to be aware of the formal aspect of the terms, but also, and often more importantly, of their interpretation. What we now characterize as normal modal logics contain intuitively plausible theorems, axioms, or definitions such as the equivalence of possibly and not necessarily not,² and the suggestion that what is necessarily the case is the case (□p → p), or equivalently in a wide class of logics, the ordinary language theorem that what is, is possible (p → ◊p).

Aquinas accepts, informally, a number of further theorems of necessity and possibility. We have, for example, the theorem (K) that if both a conditional and its antecedent are necessary, so is its consequent (□(p → q) → (□p → □q)) – that is, the necessity operator distributes over the conditional. Replying to an objection in De Veritate (2.12 ad 7), Aquinas accepts a stronger thesis, □(p → q) → □(□p → □q), which entails this one. Ed Mares has pointed out to me that, using ‘⥽’ for Thomas’s strict implication, we have (p ⥽ q) ⥽ (□p ⥽ □q), which is equivalent to the S3 axiom (p ⥽ q) ⥽ (◊p ⥽ ◊q), and Adriane Rini points out further³ that something similar is found in Aristotle: “When something false but not impossible is assumed, then what results through that assumption will also be false but not impossible.”⁴

If we add that every theorem in our system can be prefixed by the necessity operator (‘□’), we have a full fledged normal modal system, whatever our interpretation of ‘□’ may be. Aristotle accepts this necessitation rule at De Int. 19a37 and Metaphysics 1005b19,⁵ and Aquinas assumes it in a theological context at ST 1a 53.1 sed contra: “It must necessarily be said that a blessed soul is moved locally, because it is an article of faith that Christ’s soul descended into Hell.” The way in which immaterial substances move will be discussed further in Chapter 3.

Stronger modal systems may be generated by adding axioms such as the Brouwer axiom, p → □◊p, or the even stronger S5 axiom, ◊p → □◊p, either of which will allow us to deduce the equivalence of ‘g → □g’ and ‘◊g → g’, if either of these is treated as a necessary truth about g (“God exists,” for example), an equivalence which is important for certain versions of the ontological argument,⁶ but I do not know of textual evidence which shows that St Thomas accepted either axiom, though Brouwer at least would certainly have seemed plausible. Ernest Moody has suggested that the S5 axiom “is probably implicit” in the works of Jean Buridan and Albert of Saxony in the next century, but does not explain why he thinks so, apart from the fact that such an axiom is needed if iterated modalities are to collapse to a single modality.⁷

These formal points apply to all the various senses or interpretations given to the modal terms (‘necessary,’ ‘possible,’ ‘contingent,’ etc.). This distinction was clearly noted by Aristotle. At Pr Anal 25a36, for example, he remarks that “possibility (endechomenon) is used in several ways,” and on various occasions he offers what is clearly contingency as one of its senses – for example, at Pr. Anal. 32a18f he says, “I call a thing possible if when, not being necessary, it is assumed, no impossibility results.” (Aristotle has a further term available for possibility, dunaton, but in discussions of modality he treated the two terms as synonyms.) St Thomas notes that of course what is necessary is possible, but there is also a kind of possibility (“possibility or contingency”) such that the thing in question “can be, and also not be.” The contingent, that is, is “opposed to the necessary” and “excluded” by it.⁸

These formal points about Thomas’s use of modal terms are constant throughout, but there are various importantly different senses of the terms picked out. As Thomas notes explicitly, “the term necessity is used in many ways.”⁹

Thomas makes use of Aristotle’s various distinctions but, partly for theological reasons, the two have an importantly different modal vocabulary.¹⁰ Aquinas pays much more attention than Aristotle does to the distinction between necessary and contingent being – it is, indeed, his view that strictly speaking, the terms apply in the first instance to being.¹¹ He is interested in the question of how things are and must be, given their nature, a necessity he does not confuse with logical or absolute necessity. “God can do other things than those He has done.”¹²

It has already been mentioned, but it bears repeating that for Thomas, as for his contemporaries, the normal conditional was itself modalized, while the material conditional was noticed, if noticed at all, as having ‘necessity ut nunc,’ necessity ‘as of now’ in view of its content or matter (hence its name).¹³ Aquinas’s conditional may be treated, for present purposes, as if it were a necessitated truth functional conditional. It is not exactly that, perhaps, but it is at least that.

Treating the ordinary conditional as having implicit necessity (often causal necessity) avoids a standard problem regarding negated conditionals: If I deny the claim that this sugar cube will dissolve when dropped into warm water, it seems harsh to insist that I’m committed to the claim that this sugar cube has been dropped in warm water and has not dissolved. What I am committed to, however, is the result St Thomas would have accepted: There is a conjoint possibility of this sugar cube’s being dropped into warm water and its not dissolving.¹⁴

I turn now to some of Thomas’s philosophically important uses of necessity and related modal concepts.¹⁵ These uses are not sharply distinct, but even the most clearly overlapping involve important differences of emphasis.

1.2. Tensed necessity (necessity per accidens)

In general, though with exceptions, philosophers have claimed that not even God can change the past, and in De Interpretatione 9 and elsewhere Aristotle agrees. There is a certain amount of controversy concerning the correct interpretation of the De Interpretatione passages, but whether or not Aristotle intended this point at De Int. 9, he certainly accepted it in general. In the passage in which he reports Agathon as being right in saying, “For this alone is lacking even to God, / To make undone things that have once been done,”¹⁶ Aristotle treats Agathon’s claim as simply obvious.

What is perhaps our most common ordinary language use of modal terms treats them as tensed: Possibilities can cease to be, and correspondingly, necessities can arise. It was possible for you not to have read any part of this sentence, but that possibility no longer exists: It is no longer possible for you to have read no part of it; that is, it is now necessary that you have done so. As Aquinas says, “It is impossible per accidens for Socrates not to have run, if he did run.”¹⁷ This per accidens distinction was to loom large in the medieval discussions of modality, and is often associated, as in the case of Ockham in the next century, with a background modalized tense logic which views the future as branching, but the past as linear. However, Thomas, though he accepted the necessity of the past and present, and the contingency of some (but not all) claims about the future, seems to have had a view of (human) time that was linear in both directions.¹⁸

Tensed, per accidens, necessity was of particular interest to Aquinas, since for him, in view of God’s omniscience, there was a special problem about future contingents.¹⁹ Using ‘Fp’ for ‘it will be that p,’ and ‘Pp’ for ‘in the past, p,’ and restricting our variables to genuinely present tense cases (so that Fp, ‘it will be the case that p,’ and Pp, ‘it has been the case that p,’ are genuinely future and past, respectively) the problem is this: It is now true that God knows that Fp – this will be true for us, even if God is non-temporal – but in the sense of ‘necessary’ just picked out, using ‘→’ for Thomas’s conditional, both p → □p and Pp → □Pp are true.²⁰ Thus we have, given God’s omniscience, and using Kap for a knows that p

1	Fp → KgFp	God’s omniscience
2	KgFp → ▢KgFp	p → ▢p, for present tense sentences
3	▢(KgFp → Fp)	epistemic logic, ▢(Kap → p)
4	▢KgFp → ▢Fp	3, K
5	Fp → ▢Fp	1,2,4, syll × 2

For Thomas, then, Aristotle’s “there will be a sea battle tomorrow” posed proble...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Acknowledgements
• Abbreviations
• Preface
• Introduction
• PART I Natural philosophy
• PART II Natural theology
• PART III Human beings
• Bibliography
• Index