Jacques Maritain Center: Thomistic Institute

The Physical Character of Saint Thomas Aquinas' First Way

Mario Enrique Sacchi
Member of the Pontifical Roman Academy of Saint Thomas Aquinas

Many philosophers think that all the famous Thomistic five ways are demonstrations of a metaphysical character, so much so, that they only just deal with these demonstrations when they must approach the question about if there is a God in their analysis on the subject of first philosophy. Contrary to this opinion, in this paper we will try to prove that Saint Thomas Aquinas' first way is not a metaphysical demonstration, but an argument which belongs to philosophy of nature properly.

It is also important to notice that St. Thomas was not who propounded for the first time the five ways. In 1932 René Arnou, S. J., former professor at the Pontifical Gregorian University, edited an anthology where he transcribed the main demonstrations offered by several philosophers who developed those proofs before Aquinas(1). Let us restrict our considerations only to the first way. Arnou's anthology shows that Plato(2), Aristotle(3), and Maimonides(4) expounded this proof prior to St. Thomas. But, although Aquinas admitted Plato's original authorship of the first way, he affirmed that Aristotle's explanation goes on the steps of his master with the only exception of some linguistic differences in the way's formulation(5).

St. Thomas drew up the first way by synthesizing and making deeper Aristotle's sentences such as they were stated in Books VII and VIII of his Physics. Aquinas' summarized the Aristotelian theory as follows:

"The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is potentially to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, at that by another again. But this cannot go on in infinity, because then there would be no first mover, and consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands God"(6).

It is clear that St. Thomas respected strictly Aristotle's physical argument. The reasoning of the Thomistic first way belongs to the discourse of philosophy of nature. We find in this proof no resort to metaphysical apodictics, to such an extent that Aquinas' limited his statements to the speculation on the subject of philosophy of movable being. That is why the first way heads with the explicit declaration "The first and more manifest way is the argument from motion". The starting point of this demonstration confirms its physical character: "It is certain, and evident to our senses, that in the world some things are in motion". Moreover, before St. Thomas, Aristotle was convinced of the absolute priority of motion as the act firstly known by man's senses and intellect(7). Aquinas echoed the Philosopher's doctrine. St. Thomas say that motion, among all the acts, is maximally evident and patent to ourselves for we know it by sense perception. That is why motion is the first thing men call act, and from motion many other things are called acts as well(8).

The physical study of movable beings shows that the existence of motion requires both the potential state of a movable subject capable of moving in order to reach some perfection absent from its own nature, and an extrinsic cause, or a mover, which put that subject in motion. Potentiality is a conditio sine qua non of motion because everything in motion seeks perfections which it find itself bereft of. Otherwise, it would seek nothing, for the possession of perfections imply necessarily an actual, not potential, state of accomplishment. Furthermore, an extrinsic mover is also necessary to put a movable thing in motion because, as Aquinas points out, "[...] it is not possible that the same thing should be at once in actuality and potentiality in the same respect", in such a way that it is impossible that "[...] it should move itself". As we can see, this Thomistic statement also discloses the physical foundations of St. Thomas' firm rejection of any absurd causa sui.

Now, provided that "whatever is in motion must be put in motion by another", there are no reasons for going on in infinity or in an endless series of causes. All causes caused by another are effects of a preceding cause from wich they receive their moving power. However, if there would not be a first cause of motion, from which all effects in motion receive their moving power, there would be no second causes of motion, that which stands in contradiction to both the common experience and the principle of causality. It is that thus the human reasoning comes to the conclusion that the causality of a first mover, "put in motion by no other", as St. Thomas says, is absolutely necessary for movers in motion exercize their own moving causality inasmuch as they are second causes of every subsequent motion.

St. Thomas' first way has been misinterpreted by many authors who thought that his loyalty to Aristotle's philosophy of nature would be a unsurmountable obstacle to adapt it to the modern scientific understanding of the world. For instance, Étienne Gilson was of the opinion that the Aristotelian Weltanschauung, which is at the basis of Aquinas' first way, would be the main impediment for accepting it nowadays:

"The first way is presented by Thomas Aquinas as more 'manifest' because the fact of motion from which it starts is particularly evident to sense. Nevertheless, its language is disconcerting to modern readers because it is borrowed from a scientific view of the world that has ceased to be considered scientifically valid"(9).

According to Gilson, St. Thomas' first way depends on Aristotle's physical conception of the material universe. Both Aristotle and Aquinas accepted an old-fashioned universe in which

"[...] directions in space are physically real; the is a 'high' and there is a 'low'. The world is made up of four elements, with the heaviest one, earth, at the center of things. All heavenly bodies are satellites circling around the earth, each of them moved by its own mover, and the demonstration consists in showing that the number of these separate Movers must need be finite. Less strongly marked, even there, that it was in Maimonides, the presence of the Aristotelian cosmography in this [Thomistic] formulation of the proof cannot be overlooked"(10).

It is well known that Gilson's criticism of Aristotle's and Aquinas' cosmology increased more and more throughout the literary career of the French historian and philosopher. Nevertheless, on the one hand one does not find in his criticism the strict distinction between Aristotle's and St. Thomas' philosophical conception of the material world, and, on the other, their prudential respect for the theories of the ancient astronomers, cosmographers, and biologists whose opinions, given that they had no philosophical scope, were notwithstanding useful just for "keeping up appearances", so much so that neither Aristotle nor Aquinas considered those opinions as if they were apodictical inferences. On the contrary, they resulted from the investigations of a certain kind of epistemic knowledge which Aristotle attributed to that he called mixed sciences and St. Thomas scientiae mediae, i.e. that kind of sciences today comprised within the range of mathematical physics(11).

Gilson insisted constantly that Aristotle's cosmology is unsuitable for upholding St. Thomas' first way:

"[...] we today are so far removed from the universe of the Greeks that to start from the physical framework of such a universe in order to prove anything is to disqualify the whole argument [Aquinas' first way] at the very outset [...]"(12).

It is very difficult for us to know why Gilson interpreted the Aristotelian-Thomistic argument from motion just by contrasting this philosophical demonstration with the modern development of mathematical physics. At least to some extent, Gilson's position seemed to leave aside the philosophical nature of the ex motu proof, such as we can deduce it from his own words:

"That which is moving continues to move in virtue of the same property of matter by which it remains at rest unless acted upon by some external force. At the level of purely scientific explanation, which was that of Aristotle, given that bodies are already in motion, no prime mover is required in order to account for this motion"(13).

If one considers verbatim these statements, it would seems necessary to infer that Gilson rejected the first way outright because he denied the truth of its very starting point: Omne quod movetur ab alio movetur -- Whatever is in motion is put in motion by another. To say "That which is moving continues to move in virtue of the same property of matter by which it remains at rest unless acted upon by some external force", as Gilson did, implies to fall into the old mechanicist error of those thinkers who ignored the capital Scholastic distinction between the pure potentiality of first matter and the degrees of potentiality existing in every sensible body, that which is the reason why mediaeval philosophers called it materia secunda. Furthermore, in saying that "[...] given that bodies are already in motion, no prime mover is required in order to account for this motion", Gilson neglected plainly the principle of causality, for both Aristotle and St. Thomas proved that a first unmoved mover is absolutely necessary to put in motion every moved mover and every moved thing as well. It is surprising to hear from a Thomist philosopher, as Gilson was, this so grave a denial of the principle of causality such as it was stated in his last assertion.

Other defect perceived in Gilson's interpretation of the first way lies in his unfitting comparison of the philosophical status of this proof with the mechanics of modern mathematical physics. The Aristotelian-Thomistic first way arises from a process carried out according to the demonstrative method of philosophy of nature. Inversely, Gilson's objection to this proof starts from a vast prejudice against Aristotle's philosophy of nature, which St. Thomas echoed almost entirely, for Gilson thought that the Greek-Scholastic philosophy of nature would have been overcome as a whole by modern mathematical physics. In the last analysis, Gilson lessened the first way because he misunderstood the very philosophical signification of this proof.

Three conclusions can be extracted from this brief assessment of Aquinas' first way. Firstly, it is evident that the ex motu proof demonstrates apodictically the absolute necessity of a prime unmoved mover whereas all the other ones are invariably moved movers, and, at the same time, all these movers in motion receive their own causing power by participating in the causality of that first unmoved mover. Such as it has been included in his Summa theologiae, I q. 2 a. 3c, St. Thomas developed the first way as a syllogism whose logical form shows the rigorous deduction of its inference from its premises. Moreover, Aquinas did see no difficulty in assigning a divine nature to the first unmoved mover -- which "everyone understands God" -- because it is patent that such a cause of every motion is not predicated of any wordly mover. Consequently, the supra-mundane essence of the prime unmoved mover makes clear that its entity transcends entirely all the wordly beings, at least implicitly.

We can also deduce a second conclusion from Aquinas' first way, i.e. that, in addition to His supra-mundane transcendence, another four attributes predicate of God's quiddity, namely immovability, immutability, eternity, and the greatest perfection. Immovability predicates of God's nature because it is included in the first way's inference: God is understood as the prime mover which is never in motion. The same must be said of the divine immutability since a being which is never in motion cannot change, for the first unmoved mover has no passive potency to obtain any further actuality. Eternity also predicates of the prime unmoved mover because every movable thing, stricly speaking, is a sensible body which moves itself in time, but given that God is essentially the first unmoved mover, time is not an adequate measure of His duration. Finally, the prime unmoved mover's perfection is the greatest one because its immovability implies that it needs nothing, for motion is always an operation of a being in potency in order to reach perfections absent from its own essence.

Let us clarify the scope of these physical deductions of the abovementioned divine attributes. Philosophy of nature deduces them within the range of its own scientific method for they are deduced inmmediately from the very conclusion of the first way. However, the physical deduction of these attributes does not mean that the philosophy of movable being be itself a theorization on God's nature. Philosophy of nature includes no theological speculation within its epistemic range. In a noteworthy passage of his commentary on Boethius' book On Trinity St. Thomas declared explicitly that the philosopher of nature deals only with sensible things because the subject of his research is limited just to movable wordly entities, so that a being absolutely unmoved, as God Himself is, cannot be studied within the subject of this philosophical knowledge. Aquinas said that the natural science does not deal with the prime mover as it were its own subject or a part of this subject. Philosophy of nature deals with the first mover just as the terminus ad quem of its investigation, but the terminus ad quem does not belong to the quiddity of things which the philosopher of sensible being studies, although it is in a certain way related to such things. For instance, the term of a line is not a line, but its term keeps a certain relation to the line. So, even though the prime mover's nature is really different from the nature of movable things, it is related with themselves inasmuch as it is the principle of their movements. Therefore, philosophy of nature deals with the first unmoved mover in the measure that it is the cause of every motion which movable beings exercize, that is to say that such a science does not investigate the prime mover's essence, but it only takes into consideration the first mover's moving condition(14). In consequence, the physical deduction of some divine attributes does not mean that philosophy of nature be concerned with a speculation on God's nature. Philosophy of nature deduces them without getting involved expressly in an inquiry into God's quiddity, but the analysis of those divine attributes in themselves is a concern of another science.

The third conclusion reads as follows: it is presupposed in the very starting point of metaphysicians' philosophical theology that there is a prime unmoved mover which possesses certain attributes, but the consideration of these divine attributes cannot be carried out by the philosopher of nature himself, for this investigation belongs to the subject whose subject is being as such. In fact, philosophy of nature cannot advance on the content of many inferences gotten from its own scientific reasoning. Let us take for example two physical deductions. a) One of them is concerned with the demonstration of the subsistence and immortality of human soul. Philosophy of nature proves both these attributes of man's substantial form by means of its own scientific method, but it cannot find out how our soul does subsits and live in a state of separation from the human body. b) The same happens with Aristotle's and Aquinas' first way, for it demonstrates physically that there is a prime mover absolutely immovable and immutable, but philosophy of nature cannot develop a further argument to prove why both its immovability and immutability lead to deduce that God's lives a completely quiet life which is the best and the happiest one. In Books VII and VIII of his Physics Aristotle proves that there is a prime mover absolutely unmoved and immutable, but he demonstrates that God's life is the best and happiest one just only in Book Lambda of the Metaphysics:

"We say therefore that God is a living being, eternal, most good, so that life and duration continuous and eternal belong to God; for this is God"(15).

St. Thomas' first way does not resort to metaphysical reasoning because he was persuaded that philosophy is nature proves efficaciously and sufficiently that there is a prime unmoved mover. It is true that some Scholastic interpreters of this way present it as it were able to demonstrate God's universal efficient causality, or as if the proof for a prime unmoved mover would demonstate that it is the first incaused cause of everything. But this is a wrong interpretation of the first Thomistic way, for such an assumption would render the second one -- the via causalitatis primi agentis -- completely superfluous. In short, we find no reasons for seeing in Aquinas' first way a metaphysical demonstration of God because he developed his argument from motion by respecting strictly the scientific method of the philosophy of movable being.

Finally, St. Thomas' ex motu way is the very best crystallization of the Christian doctrine concerning the power of man's natural reason for understanding God starting from the knowledge of nature itself. Due to the fact that He is the first unmoved mover of every natural movement, the principle of causality allows men to rise to the knowledge of such a mover through the philosophical speculation on natural moving things.

1. Cf. De quinque viis Sancti Thomae ad demonstrandum Dei exsistentiam apud antiquos Graecos et Arabes et Iudaeos praeformatis vel adumbratis: Textus selectos, collegit et notis illustravit Renatus Arnou, S. J. (Romae: Apud Aedes Pontificiae Universitatis Gregorianae, 1932).

2. Cf. Plato, Leges X 893b-896b. See De quinque viis Sancti Thomae..., pp. 11-15.

3. Cf. Aristotle, Phys. Bk. VII, ch. 1: 241 b - 243 a 2; Bk. VIII, ch. 5 257 a 33 - 258 b 9; Metaphys. Bk. IV, ch. 6: 1071 b 3-22; Bk. XII, ch. 7: 1072 b 14-30; Bk. XII, ch. 8: 1073 a 14 - b 1; 1074 a 10-17, and a 38 - b 14. See De quinque viis Sancti Thomae..., pp. 21-41.

4. Cf. Maimonides, Dux Perplexorum, Bk. II, ch. 2. See De quinque viis Sancti Thomae..., pp. 73-79.

5. "Aristoteles, ponens omne quod movetur ab alio moveri, a Platone qui posuit aliqua moveri seipsa non dissentit in sententia sed solum in verbis" (St. Thomas Aquinas, In VIII Phys., lect. 1, n. 7). Cf. Summ. c. Gent. I 13. See De quinque viis Sancti Thomae..., p. 13, footnote 1.

6. St. Thomas Aquinas, Summ. theol. I q. 2 a. 3c, trans. by the Fathers of the English Dominican Province, 2nd ed., Online Edition 2000 by Kevin Knight. Cf. Comp. theol. I 4; In VII Phys., lect. 1, n. 4.

7. "The word 'actuality', which we connect with 'complete reality', has, in the main, been extended from movements to other things; for actuality in the strict sense is thought to be identical with movement. And so people do not assign movement to non-existent things, though they do asssign some other predicates" (Aristotle, Metaphys. Bk. IX, ch. 3: 1047 a 30-34, transl. by W. D. Ross, in The Works of Aristotle Translat-ed into English, under the Editorship of W. D. Ross, 2nd ed., 7th rpt. [Oxford: Clarendon Press, 1972], ad locum).

8. "[Aristoteles] dicit quod hos nomen actus, quod ponitur ad significandum endelechiam et perfectionem, scilicet formam, et alia huiusmodi, sicut sunt quaecumque operationes, veniunt maxime ex motibus quantum ad originem vocabuli. Cum enim nomina sunt signa intelligibilium conceptionum, illis primo imponimus nomina, quae primo intelligimus, licet sint posteriora secundum ordinem naturae. Inter alios autem actus, maxime est nobis notus et apparens motus, qui sensibiliter a nobis videtur. Et ideo ei primo impositum fuit nomen actus, et a motu ad alia derivatum est"(St. Thomas Aquinas, In IX Metaphys., lect. 3, n. 1805, editio iam a M.-R. Cathala, O. P, exarata retractatur cura et studio Raymundi M. Spiazzi, O. P. [Taurini & Romae; Marietti, 1950], p. 432a).

9. Étienne Gilson, Elements of Christian Philosophy (New York: The American Library, 1963), p. 64.

10. Ibid.

11. Cf. Juan Alfredo Casaubon, "Sobre las relaciones entre la filosofía y las ciencias positivas": Vniversi-tas (Buenos Aires) 1/1 (1967) 48-53; Id., "Las relaciones entre la ciencia y la filosofía": Sapientia 24 (1969) 89-122; and Gustavo Eloy Ponferrada, "Ciencia y filosofía en el tomismo": Ibid. 47 (1992) 18-19. See also Mario Enrique Sacchi, Eludaciones epistemológicas (Buenos Aires: Basileia, 1997), pp. 139-227.

12. Étienne Gilson, Elements of Christian Philosophy, p. 65.

13. Ibid.

14. "De primo motore non agitur in scienti naturali tamquam de subiecto vel de parte subiecti, sed tam-quam de termino ad quem scientia naturalis perducit. Terminus autem non est de natura rei, cuius est termi-nus, sed habet aliquam habitudinem ad rem illam, sicut terminus lineae non est linea, sed habet ad eam ali-quam habitudinem , ita etiam et primus motor est alterius naturae a rebus naturalibus, habet tamen ad eas ali-quam habitudinem, in quantum influit eis motum, et sic cadit in consideratione naturalis, scilicet non secun-dum ipsum, sed in quantum est motor" (St. Thomas Aquinas, Expositio super librum Boethii De Trinitate, q. 5 a. 2 ad 3um, recensuit Bruno Decker, editio photomechanice iterata [Leiden: E. J. Brill, 1965], pp. 177-178).

15. Aristotle, Metaphys. Bk. XII ch. 7: 1072 b 28-30, W. D. Ross' cited translation, ad locum.