Natural Philosophy Does Not Lead to Heresy: A Reply to John F. X. Knasas

By John G. Brungardt

In this essay, I respond to various points and counterarguments made by John F. X. Knasas in his reply to my review of his book. (In what follows, all quotations unless sourced otherwise are from Knasas’s reply.) Knasas’s main focus is my contention that it is not necessary to deploy metaphysics to defeat the Aristotelian arguments for the eternity of motion and time. In particular, he disputes the end of §4 of my review, beginning with “Absent from this analysis, however, ...”.

One way to state, in brief, the larger question at issue has already been given by Aristotle. As Aquinas puts it, commenting on Aristotle’s words, “if there is no substance other than those which exist in the way that natural substances do, with which the philosophy of nature deals, the philosophy of nature will be the first science. But if there is some immobile substance, this will be prior to natural substance, and therefore the philosophy which considers this kind of substance will be first philosophy.” (Sent. Meta., Book VI, lect. 1, at n. 1170) That is, without a prior knowledge that immobile—and thus immaterial—substance exists, one lacks a necessary condition for the initiation of metaphysics.

Knasas’s disputed point is adjacent to this debate. In view of Aristotle’s arguments that time, motion, mobiles and, thus, the world are eternal (temporally past infinite), Knasas contends as a necessary condition that metaphysical science be established beforehand for one to defuse such arguments as Aristotle proposes. By contrast, he claims, “[Brungardt] argues that the natural philosopher by his own lights can do this neutralizing, and so from the perspective of the natural philosopher the eternity of the world is not a demonstrated truth and the natural philosopher would not erroneously think otherwise.”

What is worse, “if the critique of these arguments must be metaphysical, then without metaphysics the natural philosopher is doomed to erroneously conclude that motion, and hence the world, is eternal.” This is a frequent refrain in his book, Thomistic Existentialism and Cosmological Reasoning. Absent metaphysics, Knasas thinks, natural philosophy is doomed to heresy, and such an “outcome would have someone like the saintly Aquinas teaching heresy.” (See also Thomistic Existentialism and Cosmological Reasoning, pp. 195, 255, and 217: “If there were no separately established metaphysics, then natural philosophy would lead to heresy.”)

It is true that I contended and do contend that the natural philosopher is in principle able to see that some Aristotelian arguments for a temporally past infinite world are inefficacious. I deny that the natural philosopher is doomed to conclude to the eternity of the world absent an actual habitus of metaphysics. In what follows, I first address some minor points and other items in Knasas’s reply, then I refute his more substantive points and sustain my own position, and, lastly, I conclude with a reflection on necessary and sufficient conditions. What does it mean to adequately begin metaphysics as one of the habitus among the speculative habitus of the intellectual life?

1. A Minor Correction and Initial Thoughts

First, a correction. Knasas asserts that my arguments lead to the result that “in these ways the natural philosopher by his own lights would discern the possibility of an absolute beginning of time and of existence.” In my review of his book, I neither asserted such a conclusion nor does it follow from what I did write. The former is clear by reading what I wrote, while the latter is easy enough to see in general. If someone proposes a proof that “Every S is P,” and then another points out a logical fallacy in that proof, this does not necessitate that one either has ready to hand, or even that it is possible to make, an argument proving that “No S is P” or that “Some S is not P.” In other words, by showing that certain arguments concluding “Motion is without temporal beginning” are fallacious does not commit one to the position that a certain argument is either possible or extant that concludes to the opposite.

Indeed, this agnosticism is consistent with St. Thomas’s famous “antinomy” concerning the medieval dispute about the world’s eternity. He had reasons to hold as he did that it is not possible for reason to prove either that the world is temporally past infinite or that the world is temporally past finite. Some of these reasons are theological, others are metaphysical. Other arguments that help to see the undecidability of the question to reason are themselves made via the habitus of natural philosophy and the proper use of one of its tools, namely logic.

So, while I grant the truth of the conditional statement Knasas makes (“...if the critique of these arguments must be metaphysical, then without metaphysics the natural philosopher is doomed to erroneously conclude that motion, and hence the world, is eternal”), I deny that this is a necessary condition. Rather, metaphysical critiques are a sufficient condition to defuse Aristotle’s arguments, yet other critiques are possible. Indeed, as I will reiterate below, Aquinas himself makes such critiques.

Another initial thought concerning this topic. If it is the case that Aquinas is right that the eternity of the world is a question undecidable by rational argumentation and that either one of the contradictory options is possible, then arguments which augur the impossibility of either option must either contain false premises or invalid reasoning. I contend that it is possible for Aquinas to do both: to both point out false premises and to point out fallacious reasoning. In this case, the former is accomplished through metaphysics (by arguments that it is false that the only mode of coming into being is by way of natural generation), and the latter is accomplished through the natural philosopher’s use of logic. However, Knasas, by contending the natural philosopher will necessarily demonstrate an impossibility sans metaphysics does not have such ample means of escape. His position entails either denying that there are logical fallacies in such arguments or denying to the natural philosopher the use of logic.

2. How to Beg the Question

In my review, in the section in question, I considered Aquinas’s analysis of Aristotle’s argument for the eternity of the world from the ratio of time, specifically the now (see In Phys., lib. 8, lect. 2, nn. 981–84). I then examined Aquinas’s rebuttal of this argument that it begs the question and added my own, namely, that a similar petitio principii can be found in the argument from the ratio of motion. Knasas claims (1) that I am misreading the argument and (2) Aquinas’s refutation of it, and (3) that I cannot extrapolate the rebuttal to the other arguments for the eternity of the world. None of these points are true.

First, I am not misreading the argument. Knasas contends that my “interpretation of the third argument, Aquinas says, is Averroes’ interpretation.” However, that this is not the case can be seen easily enough. Aristotle clearly makes the argument in terms of the now as an “intermediate” (n. 982) or “middle” (see 251b17; I quote the Coughlin translation). Aquinas then offers his critique that “the argument of Aristotle does not appear efficacious” and explains how it “seems to suppose the eternity of time—the very thing he ought to prove” (n. 983), namely, it is a petitio principii. Aquinas then notes that “Averroes, in trying to save Aristotle’s argument, says that the attribute of always being both a beginning and an end belongs to the now inasmuch as time is not stationary like a line, but flowing.” (Ibid.)

That is, Averroes attempts to reformulate the argument not by doing away with the “middle” character of the now but rather its “stationary [stans]” character. It is against this reformulation that Aquinas states that “this does not pertain to the purpose [propositum]” (ibid.; translation modified) and proceeds to explain how “another explanation must be furnished in accord with the intention of Aristotle.” (n. 984) As an aside, note that this progression in thinking, from the intermediate character of the now to a defense of an intermediate and flowing now is found in De Potentia, q. 3, a. 17, objs. 15–16. More on this below.

It does not seem accurate, then, to state as Knasas does that “Aquinas says the notion of the now as a middle does not agree with Aristotle’s intention.” The notion of a now as middle is precisely Aristotle’s intention, and it is precisely his intention for the very reason that Knasas takes to be in support his own reading, namely, that “the correct starting point is the understanding of the now as a beginning and as an end.” This is because the notion of the now as beginning and end is claimed to be the very identity of the now as a middle or intermediate, as Aristotle states: “the now is a certain middle, being at once a beginning and an end.” (251b20)

Aquinas himself gives us a clue here when he states that “the account of beginning and end is the same in all continua, whether they be permanent or flowing, as is clear from book 6.” (n. 983) The Leonine editors suggest Book VI, lect. 4 (perhaps n. 778, and see 233a17–21). The key point is that the physical continua of magnitude, motion, and time parallel each other in their divisibility and how points or momenta or now’s are taken in them (among many places, see 231b20 and n. 758). Elsewhere, St. Thomas points out how the “now” is both a beginning and an end:

In like manner, in the counting of motion (which counting is done by time), that which distinguishes the before and after of time is the now, which is the end of the past and the beginning of the future. Thus, the now is related to time as the mobile is to motion. Therefore, also, by commuting the proportion, we get that time is to motion as the now is to the mobile. (n. 585; and see n. 591)

That the now is “the beginning of the future and the end of the past” is true of the now as subject but not part of the now’s very ratio or nature. This is a crucial point. Were it part of the now’s very nature to be a middle, Aristotle’s argument from the now as middle would work of necessity. As St. Thomas, notes, Aristotle derives this identity of the now as middle “from what he had first supposed, namely, that prior and subsequent would not exist if time did not exist. For he uses this principle, which he supposes for no other purpose, but deduces from it that every now is a beginning and an end.” (n. 984)

Yet to make such a supposition is to beg the question! Thus, as to the second point, I am not misreading Aquinas’s refutation of Aristotle’s argument. It is precisely because Aristotle takes the before and the after as a supposition to purportedly derive the necessarily “middle” character of the now that, as Aquinas states in his exposition, the argument begs the question:

For the now is to time as the point is to the line, as was explained in book 6. But it is not necessary that a point be an intermediate, for some points are merely the beginnings of lines and others the ends, although every point would be both a beginning and an end if the line were infinite. One could not, therefore, prove that a line is infinite from the fact that every point is a beginning and an end. Rather, it is the other way around: from the fact of a line’s being infinite, one would go on to prove that every point would be both a beginning and an end. Accordingly, it also appears that the claim that every now is a beginning and an end is not true unless time is assumed to be eternal. Therefore, in assuming this as a middle term (namely, that every now is a beginning and an end), Aristotle seems to suppose the eternity of time—the very thing he ought to prove. (n. 983)

The text seems clear enough. Knasas claims at one point that “In [Knasas’s] opinion, however, since [Brungardt’s] interpretation of the third argument was incorrect, then [Brungardt’s] interpretation of Aquinas’ critique cannot be extrapolated to criticize the first two arguments for the eternity of motion.” However, as shown above, my interpretation is the correct one; or, rather, Aquinas’s interpretation is the correct one. (As an aside, even were I incorrect in the reading of the passage, I would still maintain that Aristotle’s argument begs the question and that such a critique can be extended to the other arguments; the philosophical points are independent of the interpretive one and as such Knasas cannot sustain his above criticism.)

Finally, as to the third point, one can indeed expand this argument to apply to motion and mobiles (thus, covering the other argumentation Aristotle offers in Physics, VIII.2). This is because time is the measure of motion and thus the mobile, and, as already noted, there is a “mapping” without identity of nature between the physical continua of time, motion, a quantity of the mobile and its possible locations. This mapping is extensively explored in Physics Book VI, along with preparations laid in Books IV–V.

Aquinas himself seems to hint at this. When investigating the temporal past infinity of the world in De Potentia, q. 3, a. 17, he raises the argument from the “now” as middle or intermediate (a beginning and an end) in Objection 15. Here is his reply:

Since the first succession of time is caused by movement, as is said in Physics 4, it is true that every instant is a beginning and an end of time, insofar as it is true that every moment of motion is a beginning and an end of movement; this is why, if we suppose that movement neither always existed nor will always exist, there will be no need to say that every instant is a beginning and an end of time; rather, there will be a certain instant that will be only a beginning, and a certain instant that will be only an end. Hence, this objection argues in a circle [ratio ista est circularis], and consequently is not a demonstration; yet it serves the purpose of Aristotle, who employs it to attack a position, as we have said above in the body of the article. In fact, many arguments serve to rebut an opinion on account of the statements advanced by its holders and yet in themselves are not simply speaking efficacious. (De Potentia, q. 3, a. 17, ad 15)

So, even in the De Potentia, Aquinas maintains that Aristotle’s is a circular argument. Furthermore, note how he connects the reasoning about time to its foundation in motion: “...insofar as it is true that every moment of motion is a beginning and an end of movement.” This suggests that the very same petitio is lurking in a fallacious assumption about the composition of momenta in motion and in their correlative mobile subjects.

This was already my argument in the review in the section under debate. For the purposes of this response, however, recall that Aquinas states that “It also appears that the claim that every now is a beginning and an end is not true unless time is assumed to be eternal.” (n. 983) Analogously, then, the claim that every momentum is a beginning and an end is not true unless motion is assumed to be eternal. Or, again, the claim that every mobile is brought to its coming to be by a prior mover and brought to its destruction by a posterior mover is not true unless such mobile subjects are assumed beforehand to exist.

In other words, to state or claim or attempt to prove that nows or momenta or mover-mobile conjuncts are necessarily “middles” of their very nature is to beg the question. I added in my review that this arises by “fallacy of composition” to bring out how this is the case. Indeed, Aquinas’s own rebuttal that Aristotle is using circular reasoning must be able to be extended or extrapolated precisely because of the connection between the various physical continua: time, motion, and mobiles (whether as to the intrinsic dimensive quantity or the quantities which measure them).

3. Well Begun, Half Done

What is left of Knasas’s contention that “if the critique of these arguments must be metaphysical, then without metaphysics the natural philosopher is doomed to erroneously conclude that motion, and hence the world, is eternal.”? Is the natural philosopher doomed to heresy? Is it true that Knasas’s “critique means that without a metaphysics already in place, the natural philosopher can only affirm motion’s eternity”?

Knasas is right to emphasize “the stakes involved.” The dispute about the nexus between natural philosophy and metaphysics is a centuries-old one, and even more recently among Thomists a disputed point. For instance, I have previously rebutted the argument made by Fr. Philip-Neri Reese, O.P., against the “River Forest” approach to this question. Other sources to consider are 2024 master’s thesis by Rev. Br. Columban Hall, O.P., “Finding Wisdom: Why Metaphysics Depends on a Proof for Immaterial Being,” and Glen Coughlin’s 2020 article in The Thomist, “The Role of Natural Philosophy in the Beginning of Metaphysics.” I refer the reader to those sources for the wider debate. Here, I will give some reasons why the natural philosopher is not a doomed be a heretic sans an extant metaphysics as well as noting how Knasas’s reasons (in his reply) that natural philosophy cannot reach a first mover do not prove.

First, we should note that the natural philosopher, in explicating the character of motion and the movers and mobiles involved, does not assume something true as to the principles of change or the definition of motion or the characteristic relationships in action and passion between movers and mobiles and then from these derive the eternity of motion. This is because the mover-mobile relationships (for instance) are prior to time (for time is their extrinsic measure). Mover and mobiles thus do not fall inescapably into the “horizontal” sequence of causality and the arguments for temporal past infinitude that—as Knasas emphasizes so much in his book and response—lead to the heretical result. That such arguments are fallacious is coherent with this priority of mover-mobiles to time. However, I of course grant that why movers-mobiles and their momenta and now’s of motion are not always (in this cosmos) or necessarily (of their nature) middles cannot be seen by the natural philosopher’s principles.

Second, I would like to respond to a few points that Knasas makes about Aquinas’s commentary on the Physics. Knasas notes that:

Surely if natural philosophy proved the unmoved mover, Aquinas would have acknowledged it here, just as he acknowledges that natural philosophy considers the incorruptible celestial bodies. In paragraph 245, Aquinas reiterates, “Natural philosophy does not consider every mover. … And such is the moving principle which is altogether immobile and the first of all movers, as will be shown in Book VIII.”

Knasas is speaking of texts in Physics Book II and Aquinas’s commentary (see nn. 243–45) which, Knasas contends, show “that the wholly unmoved and first mover does not belong to natural philosophy but to metaphysics.” However, Knasas’s first point, that “Aquinas would have acknowledged it here,” is not a necessary expectation. Aquinas would have acknowledged it in that place if he had the same concerns as Knasas.

Furthermore, this text does not help Knasas in his debate over the division and methods of the speculative parts of philosophy and their order and proper domains, for Aquinas himself notes at the end of n. 243 that Aristotle’s division of the three philosophical studies “is taken according to the diversity of things existing outside the mind, and not according to the division of the sciences.” So, the text is not a helpful proof text in this debate.

What is more besides, that natural philosophy does not “consider” the first unmoved mover is simply consistent with the Book VIII reading which Knasas opposes. That is, I hold along with the likes of Coughlin or McInerny that natural philosophy can terminate its considerations with a first unmoved mover but does not continue on to consider such a mover in a positive scientific mode. The Physics ends precisely because it has reached its end qua scientific habitus, resolving the universal effect of motion in the cosmos to its universal cause.

This use of “consider” is clear from In Phys., lib. 2, lect. 4, n. 175, where Aquinas notes that “the natural philosopher does not consider form insofar as it is form, but insofar as it is in matter.” This means that the last form which the natural philosopher considers is the human soul: “And so, the terminus of the consideration of natural science [terminus considerationis scientiae naturalis] are forms that are, indeed, in some way separated but that have existence in matter. And rational souls are forms of this sort.” (n. 175) However, “how forms are totally separated from matter, and what they are, or even how this form, the rational soul, exists insofar as it is separable and capable of existence without a body, and what it is according to its separable essence, are questions that pertain to first philosophy.” (Ibid.) The question of the an sit of such forms is not brought up (as my position would expect, I admit!).

Third, Knasas also claims, in support of his contention that Book VIII of the Physics must be read in a metaphysical light as having to do with existence, that “Aquinas says that Aristotle is going to take his general comments on motion and apply them to things. In other words, the reasoning of Book VII does not contain the assertion that motion exists and so strictly speaking, is not a proof.” He adds that “the versions of the Book VII reasoning in the prima viae of the two Summae begin with the assertion that motion exists.”

This contains at least two errors. First, what does Aquinas mean by the assertions in n. 967? Knasas reads them as implying that Book VII lacks an existence claim about motion. However, this is undermined by Aquinas’s words in that very paragraph of the commentary: “It is therefore a common supposition in natural science that motion has existence in things. Hence, there is no need to raise this question in natural science any more than in other sciences are raised questions about the suppositions of the science.” Besides this, such an interpretation is contradicted by standard Aristotelian scientific practice, namely, that the existence of the subject of the science is assumed.

Second, Knasas’s translation seems to require that esse in n. 967 be read in light of his interpretation of the metaphysics of esse. However, it is just as well, and I suggest, sounder, to translate this as “being.” The prior books of the Physics speak about mobile being in general, and in Book VIII Aristotle begins to apply these more determinately or specifically to mobile being. Further discussion of the like determination or “applicando motum ad determinata moventia et mobilia” can be found at n. 774, n. 896, n. 958, and n. 1009.

Fourth and finally, a word about necessary and sufficient conditions for a scientific habitus. The logical conditions for the possession of such a habit are articulated in the Posterior Analytics, at least insofar as the habit can be built up by attending to and implementing the formal structures and order described in that book. The psychological conditions for a scientific habitus are described in the Ethics, especially as regards the habit of understanding. There are also various remote conditions, such as health and sufficient maturity. Other conditions will be peculiar to the science at hand. (I briefly consider other meanings of “establishing” a science in this post.) Lastly, other habitus that are scientific can also be necessary conditions for another science. For instance, a habit of natural science is a necessary condition for the habit of metaphysics. The reason for this is that one necessary condition for establishing a science is that one must know that the subject of that science exists. The universal effect, being as such, must be known to exist beforehand in order to inquire into the causes of being as such. As Aquinas states, “our intellect is not proportioned to knowing something by natural knowledge except through sensibles; and therefore it is not able to arrive at pure intelligibles except by arguing.” (In I Sent., d. 3, q. 1, a. 2, ad 2; Coughlin translation, p. 434) Such an argument can be made by natural philosophy. (For one example, see Daniel Shields’ recent book, which I review here.)

When it comes to what the natural philosopher can do, little is left him by many thinkers. Above, what I have argued is that the natural philosopher can be expected to think logically and see the fallacies in the Aristotelian arguments, even though—as I already admitted and did not deny in my review—the consideration of being from a higher plane requires metaphysics. Does this mean that the logician is rebutting Aristotle’s arguments for the temporal past infinity of the world? Yes and no. Logic provides the tools, but the natural philosopher uses them. By way of logic, the natural philosopher can save himself from ending up a heretic.