...the past is distinguished from the future as the unchangeable from the unknowable. Is this distinction to mean that the future is still changeable? We would be inclined to answer in the affirmative, because of our simple daily experiences. Our control of the future, though certainly limited in extent, is often sufficient to satisfy our needs. Planned action, based on anticipation of what the future will bring us, has enabled us to turn many of its gifts to practical use. We sow and reap; we provide ourselves with shelter; we organize human society; we build machines that facilitate our daily work.

The scientist, however, might be inclined to question the belief that the future is changeable. Being unknowable, he might argue, does not imply being undetermined; perhaps the future is as determined as the past and the difference between past and future is merely a difference between knowing and not knowing. The apparent asymmetry of time would then be only a matter of knowledge and ignorance; time itself would be symmetrical, its objective nature would be the same in the direction of the past as in the direction of the future. Such conceptions suggest themselves within the scientific approach, because science has accepted the universal validity of causality.

Causal laws govern the past as well as the future. We see them at work in past facts; but we also see them confirmed by future facts which we have correctly predicted and which have later become reality. The future is not entirely unknowable. Quite a few occurrences can be predicted. Among these are the motions of the stars, the seasons, the growth of plants, animals, human beings; and certainly death is predictable. What led philosophers to question the reality of time is the fact that some undesirable future facts — in particular, death — are predictable. Why not assume that the future is as determined as the past?

The paradox of determinism and planned action is a genuine one. I shall later analyze it in more precise terms; at the moment it may be sufficient to point out that if time has no direction, planned action appears incomprehensible. We plan to go to a theater tomorrow; but it seems utterly senseless to plan to go to a theater yesterday. Why do we make this distinction? We answer that we cannot change the past; we can only change the future. If we went to the theater yesterday, we did; and if we did not go, we did not; what we think of it today makes no difference. Why do we not argue in the same way about going to the theater tomorrow? In our behavior we express the conviction that time goes in one direction, because planning presupposes time flow. Can scientific analysis support this conviction?

Kant presents us with long arguments in favor of his conceptions. He says that knowledge results from an interaction between physical reality and the human mind, and that the human mind superimposes certain forms on the physical things in order to make them comprehensible and accessible to order. Such forms are space and time. In particular, they are called forms of 'visualization' (Anschauung), a term meaning that space and time determine the way we see things, that is, they impose conditions on our perceptions of things but have no significance for things apart from their relation to a human observer. "It is certain .. . that space and time . . . are merely subjective conditions of all our visualizations," he writes in his Critique of Pure Reason, [2nd ed. p.66] 'This applies even to the happenings in my mind, revealed to me by an "inner sense". "If I myself or some other being could look at me without these conditions of sense perception . . . a knowledge would result in which the conception of time and change would not occur."' Time flow, change, is not a property of Being, said Parmenides; and Kant agrees, adding that time is merely the form in which we experience Being.

How does Kant come to this conception? There is no doubt that his motives are related to those of Parmenides. Like Parmenides, he wanted to escape time flow. That the fear of death determines his philosophy is obvious from his insistence on immortality. But this is not his only motive. He wanted to save freedom of will, the possibility of planned action. Parmenides was not interested in action, which presumably did not appear important to him. But Kant wanted freedom of action; perhaps not so much in order to ensure for man the possibility of improving the world, as in order to make man responsible for what he does. He wanted to establish morality; and how can human action be morally evaluated if it is not free?

Unlike the ancients, Kant faced a deterministic physics of great perfection. Newton's theory of gravitation, embodying conceptions of space and time, had greatly impressed Kant. He saw that determinism offers difficulties for human freedom, so he looked for a way out. He believed that he had discovered a way to save freedom by making time subjective — by interpreting the flow of time as the form in which we human beings experience reality, which in itself is not controlled by time. He thus teaches the ideality of time. In his denial of a realistic interpretation of time, Kant repeats the position of Parmenides; but his argument is derived from scientific developments which the ancients could not have imagined. In fact, if determinism, as formulated in Laplace's simile of a superman, holds, then time is subjective. There can be no time flow if the future as well as the past could be present to a superman's eyes. Kant saw correctly that determinism leaves no alternative. But Kant believed that he still could save free action; and the solution he offers us is to trade in the reality of time flow for the freedom of the will.

In order to understand the solution of the freedom problem proposed by Kant, we must include his treatment of causality in our consideration. Kant saw very well the close relationship between time and causality. He saw that we discover time order by examining causal order. The mere sequence of perceptions, he argues, does not establish a time order of physical happenings; in fact, we may perceive events in the inverse time order of their actual happening. "Mere perception leaves the objective relation of consecutive phenomena undetermined". For instance, even though the first sound impact at the site of a distant explosion preceded the light flash, we would see the light first and hear the sound later. To infer temporal order, we must know causal order.

Now for Kant causal order is as subjective as time. It is one of the categories by means of which we order our experience, but it does not express a property of things-in-themselves. Therefore physics can only tell us how the [phenomenal] world appears to us, not how it is, independent of a human observer. And determinism refers only to the world of appearance — the [noumenal] world in itself is free from the rule of causal laws.

This is a strange doctrine. In order to make time flow subjective, Kant has to make even causality subjective. In order to save freedom and morality, he has to sacrifice physics as a science of objective things; it is for him merely a science of experienced things. He goes beyond Parmenides in exempting Being not only from time flow, but also from causal determination.

It is difficult to understand in what sense Kant could claim that his theory reestablished freedom. When we act, we want to change those occurrences which take place in time; we want to change the future. But temporal happenings, Kant tells us, are subject to causality and determinism. What kind of influence, then, can we have upon them? It does not help us to assume that behind the experienced things there are other things not controlled by physical laws. We want to change those things that are subject to causality and are perceived in our experience; we even use causal laws to control them, knowing very well that if things did not conform to such laws we would be hopelessly lost in our attempts to plan the future. What is a philosophy good for, if it evades answers to questions about what men can do, by telling us that there is another realm of Being which we certainly cannot control? Kant's philosophy of subjective time and subjective causality is a form of escapism. It does not solve the paradox of freedom and determinism; it does not clarify the experience of time flow; it cannot account for the distinction between past and future, between the unchangeable and the realm of what we hope can be changed.

The distinction between the past and the future, the objective interpretation of time as a process of Becoming and not merely a form of human experience, has strong support in common sense. It would not be easy to acquiesce in a philosophy which regards such conceptions as illusions. Yet the convictions of common sense cannot be accepted without criticism by the philosopher. The problem of time cannot be solved by an appeal to intuitive knowledge, which tells us that there must be a process of Becoming, making planned action possible. Reliance on so-called intuition has too often turned out to be misleading. There is such a thing as an escape into common sense as well as an escape into metaphysical speculation. Neither of them offers answers acceptable to those who look for an unprejudiced approach, guided by logical analysis in combination with observation.

The conclusion of this brief survey of recent developments is that quantum physics does not present us with the time direction denied to us by the atomic occurrences of classical physics. Quantum physics worsens the situation; it makes even time order, not only time direction, a statistical property. Time appears to be a completely macrocosmic phenomenon, which cannot be traced into the microcosm; it is born anew at every moment from the atomic chaos as a statistical relationship. Strangely enough, this origin from disorder does not make macrocosmic time inferior. On the contrary, it will be seen that the birth from an atomic chaos endows the statistical cosmos with a time of exactly those properties which common sense and everyday experience have always regarded as intrinsic characteristics of temporal flow.

The distinction between the indeterminism of the future and the determinism of the past is expressed in the last analysis in the laws of physics. This is the important result of the combination of classical statistics with the indeterminacy relation of quantum physics. The consequences for the time of our experience, that is, the time of every- day life, are obvious. The concept of becoming acquires a meaning in physics: The present, which separates the future from the past, is the moment when that which was undetermined becomes determined, and "becoming" means the same as "becoming determined".

One question remains to be discussed. What is the relation between the time of physics and the time of our experience? Why is the flow of psychological time identical with the direction of increasing entropy?

The answer is simple: Man is a part of nature, and his memory is a registering instrument subject to the laws of information theory. The increase of information defines the direction of subjective time. Yesterday's experiences are registered in our memory, those of tomorrow are not, and they cannot be registered before the tomorrow has become today. The time of our experience is the time which manifests itself through a registering instrument. It is not a human prerogative to define a flow of time; every registering instrument does the same. What we call the time direction, the direction of becoming, is a relation between a registering instrument and its environment; and the statistical isotropy of the universe guarantees that this relation is the same for all such instruments, including human memory.

With these considerations, we arrive at an objective meaning of indeterminacy. Heisenberg's relation does not express merely a limitation of human capacities of knowing; it formulates, rather, a physical law holding for all physical quantities.

In many presentations of quantum mechanics, the view has been expressed that the uncertainty derives from the disturbance of the object by the human observer, whose intervention changes the observed quantity in an unforeseeable way. Such conceptions have even been put forward with the claim that quantum mechanics defies an empiricist interpretation and requires a return to idealistic philosophies, according to which the ego creates the world, or the world at least could not exist without an observing ego. I do not think that such views are tenable. The human observer is irrelevant to the indeterminacy; it is not he, or his act of observation, which creates the uncertainty. The indeterminacy is a purely physical affair and can be stated as an objective property of the physical world, without reference to an observer.

First of all, not every observation must disturb. Instruments of measurement, including those applied in quantum physics, can be so constructed that they present their indications as figures on a dial. The reading of these figures by a human observer does not disturb the object. Now it is true that when we draw the line between object and observer in such a way that the instrument of measurement belongs to the observer's side, the object is disturbed; but we need not draw the line in this way. When we include the instrument on the side of the object, the line of demarcation is drawn in such a way that the act of observation does not disturb. It is therefore not the human observer who creates the uncertainty. The indeterminacy, which for an experimental arrangement of this kind still exists, appears, rather, as a relationship between the measuring instruments and quantum phenomena, and thus as a relation between physical objects alone.

Since measuring instruments are macrocosmic objects, we may say that the indeterminacy arises when relationships between macrocosm and microcosm are involved. Heisenberg's principle states that there is no way of determining microcosmic quantities in terms of macrocosmic quantities to a higher degree of exactness than that formulated in the inequality (1). This statement expresses a physical law, a relation between macrocosm and microcosm. Since human beings are themselves macrocosmic objects and their perceptual organs respond only to stimuli in the macrocosmic sphere, their inferences concerning the microcosm must be based on macrocosmic observations; this is the reason that human knowledge of the microcosm is limited by the uncertainty principle. We thus arrive at a very empirical interpretation of the limitation of human knowledge with respect to quantum phenomena. There is no need for a relapse into idealistic philosophies, if quantum mechanics is studied under the magnifying glass of logical analysis.