It is memory that solders together the processes, scattered across time, of which we are made. In this sense we exist in time. It is for this reason that I am the same person today as I was yesterday. To understand ourselves means to reflect on time. But to understand time we need to reflect on ourselves.
A recent book by Dean Buonomeno devoted to research on the functioning of the brain is entitled Your Brain Is a Time Machine.111 It discusses the many ways in which the brain interacts with the passage of time and establishes bridges between past, present, and future. To a large extent, the brain is a mechanism for collecting memories of the past in order to use them continually to predict the future. This happens across a wide spectrum of time scales, from the very short to the very long. If someone throws something at us to catch, our hand moves skillfully to the place where the object will be in a few instants: the brain, using past impressions, has very rapidly calculated the future position of the object that is flying toward us. Further along the scale, we plant seed so that corn will grow. Or invest in scientific research so that tomorrow it might result in knowledge and new technology. The possibility of predicting something in the future obviously improves our chances of survival and, consequently, evolution has selected the neural structures that allow it. We are the result of this selection. This being between past and future events is central to our mental structure. This, for us, is the “flow” of time.
There are elementary structures in the wiring of our nervous system that immediately register movement: an object that appears in one place and then immediately afterward in another does not generate two distinct signals that travel separately toward the brain, but a single signal correlated with the fact that we are looking at something that is moving. In other words, what we perceive is not the present, which in any case makes no sense for a system that functions on a scale of finite time, but rather something that happens and extends in time. It is in our brains that an extension in time becomes condensed into a perception of duration.
This intuition is an ancient one. Saint Augustine’s ruminations on it have remained famous.
In Book XI of the Confessions, Augustine asks himself about the nature of time and, despite being interrupted by exclamations in the style of an evangelical preacher that I find quite tiresome, he presents a lucid analysis of our capacity for perceiving time. He observes that we are always in the present, because the past is past and therefore does not exist, and the future has yet to arrive, so it does not exist either. And he asks himself how we can be aware of duration—or even be capable of evaluating it—if we are always only in a present that is, by definition, instantaneous. How can we come to know so clearly about the past, about time, if we are always in the present? Here and now, there is no past and no future. Where are they? Augustine concludes that they are within us:
It is within my mind, then, that I measure time. I must not allow my mind to insist that time is something objective. When I measure time, I am measuring something in the present of my mind. Either this is time, or I have no idea what time is.
The idea is much more convincing than it seems on first reading. We can say that we measure duration with a clock. But to do so requires us to read it at two different moments: this is not possible, because we are always in one moment, never in two. In the present, we see only the present; we can see things that we interpret as traces of the past, but there is a categorical difference between seeing traces of the past and perceiving the flow of time—and Augustine realizes that the root of this difference, the awareness of the passing of time, is internal. It is integral to the mind. It is the traces left in the brain by the past.
Augustine’s exposition of the idea is quite beautiful. It is based on our experience of music. When we listen to a hymn, the meaning of a sound is given by the ones that come before and after it. Music can occur only in time, but if we are always in the present moment, how is it possible to hear it? It is possible, Augustine observes, because our consciousness is based on memory and on anticipation. A hymn, a song, is in some way present in our minds in a unified form, held together by something—by that which we take time to be. And hence this is what time is: it is entirely in the present, in our minds, as memory and as anticipation.
The idea that time might exist only in the mind certainly did not become dominant in Christian thought. In fact, it is one of the propositions explicitly condemned as heretical by Étienne Tempier, the Bishop of Paris, in 1277. In his list of beliefs to be condemned, the following can be found:
Quod evum et tempus nichil sunt in re, Sed solum in apprehensione.112
In other words: “[It is heretical to maintain that] the age and time do not exist in reality but only in the mind.” Perhaps my book is sliding toward heresy. . . . But given that Augustine continues to be regarded as a saint, I don’t suppose that I should be too worried about it. Christianity, after all, is quite flexible. . . .
It may seem easy to refute Augustine by arguing that the traces of the past that he finds within himself may be there only because they reflect a real structure of the external world. In the fourteenth century, for instance, William of Ockham maintained in his Philosophiae Naturalis that man observes both the sky’s movements and the ones within himself and therefore perceives time through his coexistence with the world. Centuries later, Edmond Husserl insists—rightly—on the distinction between physical time and “internal consciousness of time”: for a sound naturalist wishing to avoid drowning in the useless vortices of idealism, the former (the physical world) comes first, while the latter (consciousness)—independently of how well we understand it—is determined by the first. It is an entirely reasonable objection, just so long as physics reassures us that the external flow of time is real, universal, and in keeping with our intuitions. But if physics shows us instead that such time is not an elementary part of reality, can we continue to overlook Augustine’s observation and treat it as irrelevant to the true nature of time?
Inquiry into perception of the internal rather than external nature of time recurs frequently in Western philosophy. Kant discusses the nature of space and time in his Critique of Pure Reason, and interprets both space and time as a priori forms of knowledge—that is to say, things that do not just relate to the objective world but also to the way in which a subject apprehends it. But he also observes that whereas space is shaped by our external sense, that is to say, by our way of ordering things that we see in the world outside of us, time is shaped by our internal sense, that is to say, by our way of ordering internal states within ourselves. Once again: the basis of the temporal structure of the world is to be sought in something that closely relates to our way of thinking and perceiving, to our consciousness. This remains true without having to get tangled up in Kantian transcendentalism.
Husserl reprises Augustine when he describes the shaping of experience in terms of “retention”—using, like him, the metaphor of listening to a melody113 (the world, in the meantime, has become bourgeois, with melodies replacing hymns): in the moment that we hear a note, the previous note is “retained,” then that one also becomes part of the retention—and so on, running them together in such a way that the present contains a continuous trace of the past, becoming gradually more blurred.114 It is by way of this process of retention, according to Husserl, that phenomena “constitute time.” The diagram on the following page is Husserl’s: the horizontal axis from A to E represents time passing; the vertical axis from E to A’ represents the “retention” of moment E, where the progressive subsidence leads from A to A’. Phenomena constitute time because, at the moment, E, P’, and A’ exist. The interesting point here is that the source of the phenomenology of time is not identified by Husserl in the hypothetical objective succession of phenomena (the horizontal line) but rather in memory (similar to anticipation, called “protention” by Husserl), that is to say, by the vertical line in the diagram. My point is that this continues to be valid (in a natural philosophy) even in a physical world where there is no physical time globally organized in a linear way but only traces generated by varying entropy.