Newton, instead, assumes the exact opposite. In his magnum opus, the Principia, he writes:
I do not define Time, Space, Place and Motion, as being well known to all. Only I must observe, that the common people conceive those quantities under no other notions but from the relation they bear to sensible objects. And thence arise certain prejudices, for the removing of which, it will be convenient to distinguish them into Absolute and Relative, True and Apparent, Mathematical and Common.44
In other words, Newton recognizes that a kind of “time” exists that measures days and movements: the one treated by Aristotle (relative, apparent, and common). But he also contends that, in addition to this, another time must exist: “true” time that passes regardless, independently of things and of their changes. If all things remained motionless and even the movements of our souls were to be frozen, this time would continue to pass, according to Newton, unaffected and equal to itself: “true” time. It’s the exact opposite of what Aristotle writes.
“True” time, says Newton, is not directly accessible—only indirectly, through calculation. It is not the same as that given by days, because “the natural days are truly unequal, though they are commonly consider’d as equal, and used for a measure of time: Astronomers correct this inequality that they may measure the celestial motions by a more accurate time.”45
So, who is right: Aristotle or Newton? Two of the most acute and profound investigators of nature that the world has ever seen are proposing two opposite ways of thinking about time. Two giants are pulling us in opposite directions.46
Time is only a way of measuring how things change, as Aristotle would have it—or should we be thinking that an absolute time exists that flows by itself, independently of things? The question we should really be asking is this: which of these two ways of thinking about time helps us to understand the world better? Which of the two conceptual schemes is more efficient?
Aristotle: Time is nothing other than the measurement of change.
For a few centuries, reason seemed to come down on the side of Newton. Newton’s model, based on the idea of a time independent of things, has enabled the construction of modern physics—a physics that works incredibly well. And it assumes that time exists as an entity that runs in a way which is uniform and imperturbable. Newton writes equations containing the letter t for time that describe how things move in time.47 What does this letter mean? Does t indicate time shaped by the longer hours of summer and the shorter ones of winter? Obviously not. It indicates time that is “absolute, true, and mathematical,” assumed by Newton to run independently of things that change or things that move.
Newton: There is a time that passes even when nothing changes.
Clocks, for Newton, are devices that seek, albeit in a manner that is always imprecise, to follow this equal and uniform flowing of time. Newton writes that this “absolute, true, and mathematical” time is not perceptible. It must be deduced, through calculation and observation, from the regularity of phenomena. Newton’s time is not the evidence given to us by our senses: it is an elegant intellectual construction. If, my dear cultivated reader, the existence of this Newtonian concept of time which is independent of things seems to you simple and natural, it’s because you encountered it at school. Because it has gradually become the way in which we all think about time. It has filtered through school textbooks throughout the world and ended up becoming our common way of understanding time. We have turned it into our common sense. But the existence of a time that is uniform, independent of things and of their movement that today seems so natural to us is not an ancient intuition that is natural to humanity itself. It’s an idea of Newton’s.
The majority of philosophers have in fact responded negatively to this idea. In a still celebrated, furious counterblast, Leibniz defended the traditional thesis according to which time is only the order of events, arguing that there is no such thing as an autonomous time. Legend has it that Leibniz, whose name is still occasionally spelled with a “t” (Leibnitz), had deliberately dropped the letter from his name in accordance with his belief in the nonexistence of the absolute Newtonian time t.48
Before Newton, time for humanity was the way of counting how things changed. Before him, no one had thought it possible that a time independent of things could exist. Don’t take your intuitions and ideas to be “natural”: they are often the products of the ideas of audacious thinkers who came before us.
But of these two giants, Aristotle and Newton, was it really Newton who was right? What exactly is this “time” that he introduced, managing to convince the entire world that it exists, one that works so brilliantly well in his equations and yet is not the time that we perceive?
To get out from between these two giants, and in a strange way to reconcile them, a third was needed. Before getting to him, however, a brief digression on space is in order.
WHAT IS THERE, WHERE THERE IS NOTHING?
The two interpretations of time (the measure of “when” with regard to events, as Aristotle wanted; the entity that runs even when nothing happens, according to Newton) can be repeated for space. Time is what we speak of when we ask “when?” Space is what we speak of when we ask “where?” If I ask “Where is the Coliseum?” one possible answer is: “It’s in Rome.” If I ask “Where are you?” a possible answer might be: “At home.” To reply to the question “Where is something?” means to indicate something else that is around that thing. If I say “In the Sahara,” you will visualize me surrounded by sand dunes.
Aristotle was the first to discuss in depth and with acuity the meaning of “space,” or “place,” and to arrive at a precise definition: the place of a thing is what surrounds that thing.49
As in the case of time, Newton suggests that we should think differently. The space defined by Aristotle, the enumeration of what surrounds each thing, is called “relative, apparent, and common” by Newton. He calls “absolute, true, and mathematical” space in itself, which exists even where there is nothing.
The difference between Aristotle and Newton is glaring. For Newton, between two things there may also be “empty space.” For Aristotle, it is absurd to speak of “empty” space, because space is only the spatial order of things. If there are no things—their extension, their contacts—there is no space. Newton imagines that things are situated in a “space” that continues to exist, empty, even when divested of things. For Aristotle, this “empty space” is nonsensical, because if two things do not touch it means that there is something else between them, and if there is something, then this something is a thing, and therefore a thing that is there. It cannot be that there is “nothing.”
For my part, I find it curious that both these ways of thinking about space originate from our everyday experience. The difference between them exists due to a quirky accident of the world in which we live: the lightness of air, the presence of which we only barely perceive. We can say: I see a table, a chair, a pen, the ceiling—and that between myself and the table there is nothing. Or we can say that between one and another of these things there is air. Sometimes we speak of air as if it were something, sometimes as if it were nothing. Sometimes as if it were there, sometimes as if it were not there. We are used to saying “This glass is empty” in order to say that it is full of air. We can consequently think of the world around us as “almost empty,” with just a few objects here and there, or alternatively as “completely full” of air. In the end, Aristotle and Newton do not engage in profound metaphysics: they are only using these two different intuitive and ingenious ways of seeing the world around us—taking or not taking air into account—and transforming them into definitions of space.