Physics, the study of the most basic properties of matter and energy, has always been an intimidating topic for me. I listen to physicists talk about how matter moves through space and time, or to their discussions of related concepts like force, and I often feel lost. Yet I know how important the ideas of physics and the work of physicists are – for through their studies, physicists seek to understand how the Universe itself works. For this reason, I turned with a mixture of trepidation and hope to Aristotle’s Physics, and was pleasantly surprised to find the treatise interesting and accessible.
I appreciate how systematic Aristotle always is about setting forth his terms at the beginning of a treatise, describing exactly how he plans to break things down in his analysis of an area of inquiry. In just this manner, Aristotle states at the beginning of the Physics that “we do not think that we know a thing until we are acquainted with its primary conditions or first principles, and have carried our analysis as far as its simplest elements.”
Aristotle begins by interrogating the nature of existence itself, writing that “Of things that exist, some exist by nature, some from other causes. ‘By nature’ the animals and their parts exist, and the plants and the simple bodies (earth, fire, air, water) – for we say that these and the like exist ‘by nature.’” He later adds that “Nature belongs to the class of causes that act for the sake of something”; and when he begins to discuss the concept of “necessity” in nature, one senses the beginning of the discussion of the concept of natural law – a law that is ordained by nature itself, that is absolute and unbreakable, and that pre-dates any law made by human beings. This concept, so important within the field of physics, has of course taken hold in other fields as well, being applied in a religious sense by theologians like Albertus Magnus and Thomas Aquinas, and in a political sense by political philosophers like Thomas Hobbes.
Aristotle’s initial focus on motion leads to a discussion of the infinite. “The infinite,” Aristotle says, in a surprising turn, “turns out to be the contrary of what it is said to be. It is not what has nothing outside it that is infinite, but what always has something outside it.” More specifically, Aristotle states that “A quantity is infinite if it is such that we can always take a part outside what has already been taken.” He then goes on to apply these ideas to the concepts of time and magnitude, asserting that “Time…and movement are infinite, and also thinking, in the sense that each part that is taken passes in succession out of existence”, and, by contrast, that “Magnitude is not infinite either in the way of reduction or of magnification in thought.”
I found this focus on the infinite most interesting. We’ve all seen the infinity symbol (∞); many of us, as small children, may have tried to win an argument by saying “Infinity!” or “Double infinity!” or “Double infinity plus one!” And any reader of science-fiction pulp magazines has read some variation on the story where the plucky human hero outwits the arrogant, world-conquering computer antagonist by challenging it to “find the ultimate number”; the hapless machine desperately adds and multiplies ever larger numbers, endlessly, until it breaks down or explodes, and humanity is saved, all because of infinity. Hurrah!
But thinking in depth about the concept of infinity is truly much more challenging than any of that. The Universe, for instance, is said to be infinite, but with a finite age – a paradox worthy of the koans of Zen Buddhism. Additionally, the Universe is described as expanding – yet not as expanding into something “beyond” itself, but rather as expanding intrinsically, with the scale of space itself changing. These ideas are as challenging as they are important.
From motion, Aristotle goes on to consider Place in regard to nature. There seems to be an element of relativity (so to speak) in Aristotle’s consideration of place; he relates the concept of place to the four classical elements, as when he writes that “It is not every chance direction which is ‘up,’ but where fire and what is light are carried; similarly, too, ‘down’ is not any chance direction but where what has weight and what is made of earth are carried – the implication being that these places do not differ merely in relative position, but also as possessing distinct potencies.”
Aristotle is also interested in how time relates to motion – “For time is just this – number of motion in respect of ‘before’ and ‘after.’” He later adds that time “is continuous since it is an attribute of what is continuous.” He later asks, “Will time then fail? Surely not, if motion always exists. Is time then always different, or does the same time recur? Clearly time is, in the same way as motion is.” Nowadays, we talk about time as a fourth dimension, in terms of a space-time continuum; but the question of whether time had a beginning, or will have an end, is a question that brings together the speculations of physicists and of philosophers.
Later, Aristotle discusses the nature of change – what it means when something changes. In the process, he explores the differences in meaning and implication among the terms “continuous,” “in contact,” and “in succession” Consider the points on a line: Are those points continuous, or are they in contact, or do they merely succeed upon one another? The question gets even more complicated in terms of Aristotle’s declaration that “every magnitude is divisible into magnitudes”. It then comes to seem that there no points on a line that are so precisely delineated that there can’t be points in between, on an even smaller magnitude.
This is heavy stuff, clearly – and Aristotle likes to keep his discussion on the “purest” and most abstract level possible. For that reason, I found that I was most grateful when Aristotle would use a specific, “down-to-Earth” example to elucidate an abstract concept. To explain, for instance, his idea that “a thing that is in motion from one place to another cannot at the moment when it was in motion both be in motion and at the same time have completed its motion at the place to which it was in motion”, Aristotle provides this illustration: [I]f a man is walking to Thebes, he cannot be walking to Thebes and at the same time have completed his walk to Thebes”. More examples of that kind would have been welcome.
In his discussion of motion, Aristotle states that when it comes to motion, “there must be three things – the moved, the movent, and the instrument of motion.” The requirement of a movent (meaning anything that moves or causes motion) applies even in the case of locomotion, where it might ordinarily be assumed that an animal, for example, might be causing itself to move. Truly, that concept of the movent is key to Aristotle’s ideas regarding motion. When it comes to circular motion like that of the planets – in Aristotle’s system, the only true continuous motion, since motion back and forth along a line necessarily involves some degree of stopping and starting – Aristotle states that in the case of circular or spherical motion, “the movent must occupy either the centre or the circumference, since these are the first principles from which a sphere is derived.”
It is at this point that the Physics becomes once again a treatise that might be as interesting to theologians and philosophers as it already is to physicists – as Aristotle begins positing the existence of a “first movent” that “is indivisible and is without parts and without magnitude.” This concept is important in Aristotle’s system because of his belief that “there always was and always will be motion throughout time”, and his search for “the first principle of this eternal motion” leads him to the “primary motion…the only motion that can be eternal; and we have pronounced the first movent to be unmoved.” Elsewhere, Aristotle refers to this concept as a “First Principle.”
That idea of the Greeks’ ὃ οὐ κινούμενον κινεῖ, or the Romans’ primum movens, or what we might call the “Unmoved Mover,” has been of great importance to many theologians. Thomas Aquinas, for example, made that concept of an “argument from first cause” the first of the Quinque Viae (Five Ways) through which he sought to provide logical proofs for the existence of God, as part of his Summa Theologica (1274). Everything has a cause, Aquinas reasons; and therefore, if one goes further and further back, from each cause to that cause’s own prior cause, one will eventually arise at the first cause before which there is no other cause – and in Aquinas’ system of reasoning, that first cause is, of course, God.
However you feel about Aquinas’ interpretation of Aristotle’s idea of the “unmoved movent” or “First Principle,” we can all agree that – in physics, as in so many other areas of human inquiry – Aristotle established the conversation, setting down its basic premises in terms that the thinkers of later eras would find to be an eminently reasonable basis on which to develop their own hypotheses, devise their own experiments, evaluate their own results, and draw their own conclusions. As always, it is a pleasure to visit with Aristotle, bask in the brilliance of his rigorous intellect, and watch him begin the conversation.