Hans Reichenbach on the Direction of Time
Part of a Series on the Philosophy of History
Thursday 26 September 2024 is the 133rd anniversary of the birth of Hans Reichenbach (26 September 1891–09 April 1953), who was born in Hamburg on this date in 1891.
Reichenbach is remembered as a mathematician and a philosopher of science. I don’t know of anyone who has talked about Reichenbach in relation to philosophy of history, so why in the world would I bring up Reichanbach? Reichenbach wrote two books on time, The Philosophy of Space and Time and The Direction of Time, with The Direction of Time being the last book he published. I’ve argued in several episodes that a philosophy of history ought to be based on a philosophy of time, so Reichenbach gives us a philosophy of time consistent with, or, better, based on, the science of Reichenbach’s time. Reichenbach also wrote a work of history, From Copernicus to Einstein. This is a history of the development of science from the beginnings of the scientific revolution to the early twentieth century intended for a popular audience, so we can add science communications to Reichenbach’s achievements.
Reichenbach’s scientific perspective on time points us both to the immediacy of time and to the longest periods of cosmological history. Human history, always the traditional focus of history, occupies a middle ground between the immediacy of time, intimately known to us through the experience of consciousness, and the aeons of cosmology that science has revealed to us. But it is in the middle distance of human history that scientific discovery takes place. That is to say, the human scale of time that interests traditional historians is the scale time at which a part of the universe that has evolved the ability to understand itself acquires the knowledge of the conditions of its existence and its place within the universe that produced it.
So there are two histories — the history of the universe, of which the knowing being is a part, and including the stages of the development of that knowing being, and the history of scientific discoveries that have disclosed to that knowing being how it came to be, from the earliest origin of the universe to that being’s awareness of the earliest origins of the universe. In the first history, electromagnetic radiation has been propagating at the speed of light since the origins of the universe. In the second history, the scientific determination of the speed of light occurred in 1676 (preceded by ideas about light that did not reflect a scientific understanding of the world), and the place of light within the spectrum of electromagnetic radiation was not fully appreciated until Maxwell formulated his equations that theoretically unified light and electromagnetism. So while Reichenbach sets out in his history book to tell the reader about the most recent cosmological theory, intended to tell the story fo the first history, he discusses the lives of Copernicus, Galileo, Kepler, and Newton at their human scale of time, telling the story of the second history.
We can take it as a further principle of Copernicanism that a knowing being, while partaking of the history of the cosmos entire, comes to an understanding of this relationship at the same time scale of its own knowing, which is, in turn, embedded in a social context. Human history is like the tiny portion of the electromagnetic spectrum revealed to us by our eyes, of which Reichenbach wrote:
“…when we visualize the whole scope of electrical waves and notice the little band of rays perceptible as light, it appears to us as if the world were covered with a curtain with a small hole through which we are allowed to contemplate only a fringe of nature’s immense riches.”
So too the whole of cosmological history is covered by a curtain, and human history is the small hole through which we are allowed to contemplate only a fringe of nature’s immense riches, and it is this small hole of human history through which contemplate the cosmos that is the matter of Reichenbach’s From Copernicus to Einstein.
In my episode on Haskell Fain I quoted Fain on the need for speculative philosophies of the history of science:
“What positivism lacked… was a penetrating speculative philosophy of the history of science, the encouragement to fashion story-lines upon which better histories of science could be constructed… Each kind of history requires its own kind of speculative philosophy of history.”
There is a speculative philosophy of the history of science implicit in Reichenbach’s history. On the very first page of the book we find a couple of theoretically interesting claims about history:
“Astronomy, as a science, has come to forget its primitive wonder: instead, it approaches the realm of stars with sober research and calculation. This disenchantment with its subject-matter, which scientific study invariably entails, has permeated astronomy to a greater degree than the layman realizes. In observing the astronomers of today, how they measure, take notes, calculate, how little attention they pay to mysterious speculations, one may be surprised to find the wonderful structure of learning so cut and dry at a close range. Yet nothing is more wrong and more objectionable than the feeling of a heartbreaking loss, with which some people regard the vanishing mysticism of the skies. Although science may have destroyed a few naive fantasies, what she has put in their place is so immensely greater that we can well bear the loss.”
In this passage Reichenbach has employed the idea of the disenchantment of the world in the wake of the scientific revolution, which was originally formulated by Max Weber, and the paragraph finishes with the now-familiar idea that, whatever has been lost to disenchantment, the compensations of science more than make up for it. The mythology scholar Joseph Campbell later made this claim, writing that, with science, “…we have a still greater, more alive, revelation than anything our old religions ever gave to us.” Moreover, the continuing pursuit of science promises a boundless future of exploration and discovery, together with the continual growth of the human mind.
Reichenbach’s twofold contribution to the philosophy of history, then, lies in his philosophy of time, including the application of this philosophy of time to history, and the implicit philosophy of history in his history of modern science. It’s mostly Reichenbach’s philosophy of time that interests me. In my episode on J. M. E. McTaggart I discussed what I called the historical supervenience principle, according to which a philosophy of time is implicit in any philosophy of history, and vice versa. Having thought more about this, I can distinguish four permutations of the historical supervenience principle, with each of the four permutations formulated relative to naturalism. These four are:
- Any conception of history supervenes on a conception of time.
- Any naturalistic conception of history supervenes on a conception of time.
- Any conception of history supervenes on a naturalistic conception of time.
- Any naturalistic conception of history supervenes on a naturalistic conception of time.
These four permutations are quite similar, but the differences among them, while they are subtle, are nevertheless important. Let’s take each one separately.
- Any conception of history supervenes on a conception of time.
This is the most general formulation of the idea, according to which any history involves some philosophy of time, and any philosophy of time involves some philosophy of history. Being the most general formulation, it is also the formulation with the least content to it, and in this form the historical supervenience principle is little more than a nod to implicit presuppositions.
2. Any naturalistic conception of history supervenes on a conception of time.
This formulation limits supervenience upon time to naturalistic philosophies of history, implying that non-naturalistic philosophies of history do not in fact supervene upon any conception of time, or do not need to supervene upon any conception of time, in turn implying a greater degree of autonomy on the part of non-naturalistic philosophies of history. By the same token, naturalistic conceptions of history have less autonomy because they are more tightly-coupled to the world through a conception of time.
3. Any conception of history supervenes on a naturalistic conception of time.
This formulation implies that any conception of history, whether we know it or not, whether naturalistic or not, supervenes on a naturalistic conception of time, which means that a naturalistic conception of time is at the foundation of all conceptions of history regardless of their ontological or epistemological commitments of a given philosophy. In this formulation of the historical supervenience principle, it is a naturalistic conception of time that is foundational and is expressed in all history.
4. Any naturalistic conception of history supervenes on a naturalistic conception of time.
This formulation is the narrowest, confining historical supervenience only to naturalistic conceptions of history supervening on a naturalistic conception of time. While this is the narrowest of the four permutations I have considered, in another sense it is a schema for a class of historical supervenience principles.
I’ve formulated my four permutations of the principle of historical supervenience relative to naturalism, but the possibility of formulating them relative to some other metaphysical scheme reveals another principle implicit in the above four permutations. The schema that we can derive from the final and narrowest of historical supervenience principles is this: Any such-and-such conception of history supervenes on a such-and-such conception of time. That is to say, each metaphysical scheme corresponds to a conception of time and an associated conception of history that supervenes on this metaphysically specific conception of time. Thus not only does a naturalistic conception of history supervene on a naturalistic conception of time, it is also the case that a non-naturalistic conception of history supervenes on a non-naturalistic conception of time. This is more specific, i.e., it has more content, than the first permutation, but in another sense it is the familiar idea that a philosophy of history can be derived from any distinctive metaphysical position. I’ve discussed this and closely related ideas in several episodes.
Reichenbach’s conception of time is thoroughly naturalistic, and intentionally so. Moreover, Reichenbach’s naturalism is the early twentieth century naturalism of logical empiricism, and he is keen to provide a rigorously scientific basis to his account of time. Despite the radicalism implicit in Reichenbach’s logical empiricism, his history and his philosophy of time are still traditionalist in many ways. Reichenbach’s history From Copernicus to Einstein is a story of larger than life figures — Copernicus, Galileo, Kepler, Newton, and Einstein. Reichenbach several times calls Einstein the Copernicus of our time (i.e., his time), making Copernicus a kind of historical exemplar of the scientist whose type only rarely appears in history, and which was represented in Reichenbach’s time by Einstein. And Reichenbach ends From Copernicus to Einstein on a rhetorical flourish that invokes Schopenhauer:
“…just as the Copernican worldview became at last generally recognized and a common property of all educated people, so will it be with the theory of relativity. One hundred years from now, the doctrine will be accepted as self-evident; and it will be difficult to comprehend why it encountered at first so such opposition. In Schopenhauer’s words, ‘Truth is allowed only a brief interval of victory between the two long periods when it is condemned as paradox or belittled as trivial.’ We who are permitted to see this period of victory with our own eyes may consider ourselves fortunate to witness the Copernican discovery of our age.”
Reichenbach practiced what Nietzsche called monumental history, and we have seen in the episode on Thomas Kuhn that the tradition of monumental history has been on the back foot, with recent historiography of science focusing on the fine detail of scientific discovery rather than the great figures of science.
Reichenbach’s conception of time is also traditionalist in his references to the flow of time. Many recent philosophers have criticized the description of time as flowing as being a mere metaphor, as they also criticize the metaphor of time as a river. In The Direction of Time, Reichenbach begins his exposition with what he calls a qualitative account of time, which will later be contrasted to his quantitative causal theory of time, but in the qualitative description of time Reichenbach does not hesitate to say that time flows. Of course, many philosophers have explicitly acknowledged that our language falls short when it attempts to describe time.
This problem is the point of departure for one of the most famous discussions of time in philosophy, the 11th book of St. Augustine’s Confessions, which begins with the well-known assertion that when no one asks Augustine what time is, he knows perfectly well what it is, but when someone asks him, he doesn’t know. Many other philosophers since Augustine have struggled with the problem of giving a rational account of time. Reichenbach’s elder contemporary Husserl wrote hundreds of pages attempting to describe time as it is experienced, and while other phenomenologists have taken up this problem also, non-phenomenological philosophers have actually criticized Husserl’s account for being too detailed. Reichenbach, like Husserl, begins with an attempted experiential account of time, even discussing what he calls the emotive meaning of time, but he quickly pivots to physics, because what he wants to do is to give an account of the directionality of time on fundamental physical grounds, not on experiential grounds.
We can understand Reichenbach’s willingness to employ the inexact language of time flowing as a concession to our intuitions about time, so that he can get past this informal account of time and get to the real meat of his theory, which is a physics-based causal account of time. We all share this dilemma in thinking and talking about time, and the language we use in the attempt to formulate a theory of time is what Bergson called spatialized. Visualizing time as a river in which the same man cannot step twice, because the second time it is not the same man and not the same river, is a spatialization of time, in which we attempt to get control of the flowing aspect of time by likening it to the length of a river, which flows from its source to an ocean. If our conception of time is based on our conception of space, even if only indirectly, this facilitates reductionist theories of time in which time is nothing but one dimension of a four dimensional continuum in which our present experience is one slice through that continuum.
The problem of understanding time on its own terms is carried through to philosophy of history in the problem of historical observation and evidence. The historian can never observe history, but only the traces that past actuality has left in the present, because past time has disappeared as soon as it has ceased to be present. This a problem that interests Reichenbach, and he gives a distinctive account of it that has few parallels in the philosophical literature.
Reichenbach formulates his opening qualitative account of time in five statements:
Statement 1: Time goes from the past to the future.
Statement 2: The present, which divides the past from the future, is now.
Statement 3: The past never comes back.
Statement 4: We cannot change the past, but we can change the future.
The formulation of his fifth qualitative property of time is more guarded, and is stated with conditions. Reichenbach begins by saying, “We might be tempted to formulate another difference by saying that the future is unknown, whereas the past is known.” However, he says that this is obviously false because science does give us knowledge of some future events through prediction. Eventually he converges on another formulation of essentially the same intuitive idea, which is:
Statement 5: We can have records of the past, but not of the future.
And he concludes with:
Statement 6: The past is determined; the future is undetermined.
Reichenbach’s hesitation over the formulation of his fifth qualitative property of time is relevant to the whole argument of the book, since he attempts to show how and why we have records of the past, but not of the future. In order to demonstrate this, the bulk of the book is about thermodynamics, which Riechenbach employs as an irreversible physical process from which the directionality of time can be derived. Reichenbach employs the intuitive example of footprints in -sand as a way to illustrate his argument:
“Suppose we find in the sand traces of footprints, somewhat smoothed out by the wind, but still recognizable as impressions of human feet. We conclude from this ‘record’ that at some earlier time a man walked over the sand, thus causing the footprints. What is the logical schema of this inference?”
It is interesting that Reichenbach uses the example of footprints. In 2022 the PBS series NOVA aired an episode title “Ice Age Footpints” in which human footprints discovered at White Sands, New Mexico, were shown to have been dated between 21 and 23 thousand years before present. The science demonstrated in this NOVA episode is characteristic of the kind of scientific history that has significantly expanded our knowledge of human history since the second half of the twentieth century.
The inquiry implicitly anterior to the work described in this NOVA episode is the kind of work that Reichenbach has done in his The Direction of Time. Reichenbach gives us the deeper substructure of this scientific argument. Reichanbach’s whole argument, based on a thoroughly scientific conception of records, can be applied more generally to history. The traditional historian confined his efforts to written documents, which are records of the past no less than a footprint. A scientific conception of records, as we find in Reichenbach, does not distinguish, at the resolution of this argument, between documents and footprints.
