Bernard d'Espagnat (1921-2015), in a way quite untypical of most contemporary scientists, and renewing the French tradition in natural philosophy, has done philosophers a service by making fully explicit the philosophical consequences of one of the main results of quantum mechanics (QM), the discovery of non-separability or non-locality. This is the idea according to which two objects, e.g. two correlated electrons (having the same associated wave) seem to continue to influence each other after separation. In other words, where we thought that there were two systems, maybe we should imagine that there is actually only one. He has extended his scientific work by a series of comments on some recent views on the nature of science and its relations with philosophy, giving us an inside look at the way actual science works. What I will say does not concern his entire work but only some of the main points directly connected with his main philosophical thesis that reality is veiled. To do this, I consider his books and some letters we exchanged on this problem.
In a nutshell, d'Espagnat's thesis is this: aware that there are long lasting rival theories aiming at describing reality-in-itself (physicists seem to agree that, as far as the description of the empirical world of phenomena, QM has no rival), he thinks that it is useless to ask them to describe anything beyond phenomena. That is why, he argues, it is likely that reality-in-itself is not scientifically knowable, that is, in a way which is both certain and unambiguous. The proper object of science is thus empirical reality only directly or indirectly described by statements endowed with weak objectivity. This remark adds weight to the ancient relativistic and anti-realistic belief that it is hopeless to wait for a unified metatheory that would be the final description of a unique reality-in-itself.
The global suggestion made by d'Espagnat is that there are some reasons, some facts, some a posteriori evidences which invite us to believe that there is not an ultimate reality, knowable, yet independent of the human way of perceiving and thinking. Such a reality is veiled. A veil is exclusively an obstacle to sight. A veiled object can be touched, smelled, and if the veil happens to be close to the surface of the object, we can even guess its form, and eventually, its nature. The veiled reality metaphor means then that there is a reality-in-itself. That is why sometimes the doctrine that reality is veiled is considered ―misleadingly― as a kind of realism. Misleadingly at least to the extent in which often people go on to postulate that the order of things, their intelligibility, come from our minds, which is actually a definition of idealism. From the veil metaphor it does not follow that reality does not exist: d'Espagnat has criticized positivists and empiricists for thinking so. The philosophical lesson of QM is epistemological, not ontological. Reality, epistemologically veiled, can be “touched”, it is not absolutely beyond our capacities.
It would be difficult to find a philosophy better adapted to the teachings of QM than Kant’s. I would like then to call attention to the fact that, for d’Espagnat, reality is not absolutely unknowable, as it was for Kant. According to the physicist, once in a while the scientist gets the impression that he has grasped reality-in-itself, a part of its structure. But reality veils her face, so the scientist has a difficult time in trying to say what is the part of reality he believes to have perceived. He draws a lesson of modesty and tolerance: maybe this feeling of having touched reality is also present in other people having devoted their lives to a non-scientific endeavour such as art or religion. After all, science does not pretend to exhaust the aspects of reality. Science has often been partially defined, by scientists themselves, as the description of phenomena.
The situation is confusing for the philosopher of nature when the specialists of a given theory do not agree about the facts established, nor about the right or more plausible interpretation of them. It is not comfortable to comment on the supposed evidences coming from QM. Here reality is conceived as that which exists beyond sense impression, and it is pertinent to remember that physicists have a tendency to distinguish neatly between reality extensively conceived as the set of subject-independent physical objects, and a subjective or intersubjective world, the world of perception. The subjective world seems to be here a cellar where the physicist throws everything he cannot or does not want to use.
One of the first things to say then is that there is no need to see reality that way: man, and what exists in his interiority —with the exception of what exists only intentionally, within language— is also real; his sense impressions, sensations, and perceptions are real. Is real then, so to speak, what is in both sides of sense impression. The real photons or waves which exist beyond sense impression come into the organism. Colour vision is the set of those photons or waves in the organism. Thus those real events, i.e. those physical changes in the state of something, are not the cause of sensation, if by cause we mean that which exists exclusively before the effect: they are the substance of sensation. Colour vision is the existence of those physical entities in the organism. The same can be said concerning the other kinds of sensation. This assertion is not refuted by the observation that different sets of physical properties can be integrated in the vision of (roughly) the same colour: a blue ocean and a blue sky are blue for different physical properties.
If, as I suggest, the real, physical events are not transformed when they come into the organism, if they are not just the cause which disappears after they produce the effect in the organism, it follows that the events entering into the composition of the thing in the intellect, in the object, as well as those parts of the state of the organism or of the central nervous system which do not exist only intentionally, are real parts of being. The Ancients were right: phenomena tend to reveal the properties of reality, whereas for many modern thinkers, on the contrary, phenomena hide reality and reveal the properties of the organism. These properties veil reality. The problem then is not to go out to reach things as they are since we are part of reality; or, if you wish, reality in the form of physical events existing on the one or the other side of sensation, goes through us, whether we like it or not. Reality is not a special area of the universe, that area which exists beyond the reach of our senses. I hope that some day neurophysiologists will be able to propose a theory of sensation in accord with this idea.
I think that d'Espagnat traces too sharp a distinction between science and non-science, and between phenomena and reality-in-itself. Why should science be the description of phenomena? Why should reasoning from bottom to top be preferable to beginning from the other end? Much of what he says follows logically from this conception of science and this conception of the best reasoning. On the contrary, if the aim of science is the search for intelligibility, and if intelligibility is not equivalent to the result of empirical research, one sees how considerations other than those tied to reasoning from bottom to top may be pertinent. If, as I think, intelligibility has also a metaphysical side, and if we associate this aspect to the summit of the intellectual process, reasoning from top to bottom looks as legitimate as the other way. There is no research without a priori principles. For instance, according to my metaphysical realism, two such principles are the belief that nature is thinkable, and the belief in causal determinism. Without them the scientist would have nothing to do. In other words, if science and metaphysics do form one conceptual, logically coherent speculative system, there is no reason to trace a sharp boundary between phenomena and noumena.
Kant has sharply separated phenomena from noumena, science from metaphysics. It is not surprising then that some people go on to deny the world of noumena: if it is absolutely unknowable, we cannot say anything about it, we are not even entitled to say that it exists. This shows the difficulty to acknowledge the existence of something about which we do not know anything. D'Espagnat, conscious of the difficulty, does not want to commit himself to the view that reality is absolutely unknowable, and, as I said earlier, he finds in this a difference between his thought and Kant's. But I do not think his reaction does away with the feeling of uneasiness awakened by the recognition that sometimes the scientist believes to have touched reality, although he is unable to say exactly what he has touched. It is as if man were surrounded by shadowy figures, like those of Plato's allegory, living in a world devoid of substance and light.
How can we know that we have touched reality-in-itself? According to my metaphysical realism, when we obtain approximately the same results no matter how we investigate phenomena. Identity, invariability, or rather, stability, is a reliable criterion of reality. Of course identity has not necessarily to be taken in its artificial symbolic, mathematical or logical sense which states that identity implies absolute immutability or exact numerical precision. Here a reasonable attitude is required, I mean for instance, the kind of attitude that makes us recognize that an event is determined when we are able to predict what kind of event will follow from such and such an event (at home we know that our cat will come close to us when we sit down to read in our sofa, even if we are unable to tell exactly, in minutes and seconds, when she will come and exactly what surface of the sofa she will occupy). And if this criterion of identity, of multiple verification, is not a criterion of reality, I do not see how any other criterion would succeed. It is the business of each theory or science to specify its criteria for identity according to its own nature. Thus if QM has been confirmed again and again, its value is more than conceptual coherence: it goes beyond theoretical harmony to touch reality.
Maybe d’Espagnat would have agreed to this, but he would have added that we do not know what it is that we touch since, for him, reality was veiled. There are then two opposite ways of understanding the role of QM: according to the orthodox interpretation, shared by d’Espagnat, it makes it definitely impossible for us to know things as they are. According to another interpretation, in agreement with the idea that the highest objective of science is precisely to discover how things are even beyond the reach of our senses, it could be said that this theory is incomplete; that it does not exhaust all man can rationally learn about the quantum physical world. What is regrettable in most scientists, and d’Espagnat is not an exception, is their unwillingness to continue thinking rationally and imaginatively beyond the information coming, at a moment, from experimental science.
The picture one gets from d'Espagnat's thesis —reality is veiled— is not reassuring, although we must recognize that after all the physicist, as physicist, cannot do any better but end up by saying that, as far as he knows, reality is veiled. Now both the natural philosopher and the mathematician feel uneasy, and since for them empirical science does not have the last word, they keep on thinking imaginatively and rationally, employing their proper talents and tools, in order to unveil reality.