The Principle of Sufficient Reason is crazy. But what is the alternative?
Let’s see. If every event has a cause, then whatever caused it was itself caused by other events, which in turn had their own causes, and so on. Every event is tied to other events, in an unbroken chain that must go back to the beginning of time. Everything that ever happened, and everything that will ever happen, was and will be destined to do so, according to the great laws of nature. Events are bound to obey those laws. Laplace’s demon, who knows them, can foresee everything: ‘the future, as the past, is present to its eyes’. Or, as Albert Einstein famously put it in a letter of condolence to the family of his lifelong friend Michele Besso, less than a month before his own death: “the distinction between past, present and future is only a stubbornly persistent illusion”. Einstein loved to discuss the nature of time with his friend Kurt Gödel over long walks outside the Institute for Advanced Study in Princeton. In general relativity, space-time is a four-dimensional manifold, a block universe where everything exists in a tenseless present. In the words of Hermann Weyl: “The objective world simply is, it does not happen”. Gödel approved.
If this is crazy, let’s try the opposite. Events are not tied together. They are free to happen. They are nothing before they become into existence and nothing after the cease to exist. Therefore, Laplace’s demon does not know them and cannot foresee them. Nature is not governed by laws. What we call natural laws are satisfactory local explanations, verbal acts that we share through language and agree to accept.
At first, this sounds as crazy, if not crazier. Surely, the ball is not free to land on the floor. The fact that it always behaves in exactly the same predictable way must mean that it is subject to a certain law, which we discover by experience but has nothing to do with us: it is there, whether or not we express it and accept it. How we express it may change through time: Aristotle’s natural place, Newton’s law of universal gravitation, Einstein’s theory of general relativity. But what it is must be timeless.
What does ‘there is a law’ mean, however? What is there, what we experience are not laws but events. An event is what happens, comes out into existence. Another word for it is phenomenon, from Greek phainesthai, meaning ‘to appear’, from phaos – light, the same root as in ‘philosophy’. But there is a difference: while a phenomenon appears, comes to light from where it ‘simply is’, an event comes out of nothing. It is nothing before it exists and therefore cannot be foreseen. As Hume pointed out, what we see are not laws but events, and their more or less constant conjunction – today we say higher or lower correlation – with other events. This is what we call experience. An experiment is an orderly gathering of what we see – evidence – in order to properly measure such correlation. But, as is (or should be) well known, correlation does not imply causation. More correctly, covariation is a necessary but not sufficient condition for causation. What is needed to turn correlation into causation is a satisfactory explanation. In the overstated words of Friedrich Nietzsche:
Against positivism, which halts at phenomena – “There are only facts” – I would say: No, facts is precisely what there is not, only interpretations. We cannot establish any fact “in itself”: perhaps it is folly to want to do such a thing. (Will to Power, Fragment 481 (1883-1888)).
(Incidentally, Nietzsche would have also been greatly helped by a Word Processor. Unlike Wittgenstein, he wrote and published many books, but had such little regard for order and consistency that he ended up being dismembered by his own interpreters – as one of them, my friend Sossio Giametta, puts it in Il Bue Squartato (The Quartered Ox). Contrary to Nietzsche’s hyperbole, Giametta has done a great job, over a lifetime, in establishing the key facts of Nietzsche’s life and work, and built on them what I think is the most complete, well-rounded and incisive interpretation of his philosophy).
Nietzsche went too far. There are facts – events, evidence, information, data. But he was right in saying that facts are not enough by themselves: they need to be interpreted, i.e. embedded within an explanation. We read facts, but we write explanations. And often our writing includes choosing the data on which our explanations are built. At the same time, however, explanations are interpretations of facts: there cannot be explanations without facts to be explained.
What there is, then, is evidence, which we experience, not explanations, which, as Bruno de Finetti put it, we invent. There where? In a timeless universe, evidence consists of the coming into light of eternal phenomena, governed by the great laws of nature in accordance with the Principle of Sufficient Reason. Alternatively, evidence consists of events, coming out of nothing and going back into nothing. In such a universe, far from being a stubborn illusion, time is the essential dimension along which events unfold. As Lee Smolin puts it:
Whatever is real in our universe is real in a moment of time, which is one of a succession of moments. (Time Reborn, xiv).
Smolin’s book is a valid attempt to move our perspective away from a timeless universe to a physical world where:
The past was real but is no longer real. We can, however, interpret and analyse the past, because we find evidence of past processes in the present.
The future does not yet exist and is therefore open. We can reasonably infer some predictions, but we cannot predict the future completely. Indeed, the future can produce phenomena that are genuinely novel, in the sense that no knowledge of the past could have anticipated them.
Nothing transcends time, not even the laws of nature. Laws are not timeless. Like everything else, they are features of the present, and they can evolve over time.
It is puzzling, therefore, to find that one of the cornerstones of Smolin’s proposed cosmological theory is none other than the Principle of Sufficient Reason (p. 122), which he customarily attributes to Leibniz rather than, more correctly, to Spinoza. How can that be reconciled with the reality of time, the unreality of the past and the openness of the future? Unsurprisingly, in the sequel to Time Reborn, written together with Roberto Mangabeira Unger, his co-author disagrees:
We cannot show, as the principle of sufficient reason would require, that the world has to be what it is or that it has to be at all. (The Singular Universe and the Reality of Time, p. 514).
This gives Smolin the opportunity to adjust his focus and reinterpret the principle as a ‘heuristic guide to suggest questions to ask and strategies to follow, in the formulation of hypotheses for cosmological and physical theories’ (p. 514). He calls it ‘the principle of differential sufficient reason’:
Given a choice between two competing theories or research programs, the one which decreases the number of questions of the form Why does the universe have property X? for which we cannot give a rational explanation is more likely to be the basis for continued progress of our fundamental understanding of nature (p. 367).
That is: better a theory that answers more whys than a theory that answers fewer. Oh well. A truism more than a principle – and very far from Leibniz’s bold claim. But Smolin is right. Contrary to its original meaning, a theory is not the contemplation of a revealed truth. It is a local explanation that gives a satisfactory answer to as many questions as it can.
Like beauty and evidence, however, satisfaction is in the eyes of the beholder. A few years ago, while they were still enthralled by Santa Claus, I asked my children: How can Santa, who is so fat, come down the chimney? They looked at me with mild contempt, shrugged their shoulders and said: He’s magic! As a counterbalance to their incessant inquisitiveness, children have a penchant for the almighty. Magic means to be able, to have power, from the Proto-Indo-European magh-, from which the noun might and the verb may come from. There is a perfectly satisfactory answer to why Santa comes down the chimney: because he can. In the same vein, our primordial ancestors had Santas for every noticeable natural phenomenon, and many fanciful strategies to propitiate their favours. As they grew up, however, curiosity prevailed. The history of science is the steady unfolding of natural explanations superseding magical shortcuts.
As Smolin says, a persistent search for more encompassing answers to our why questions is a powerful heuristic guide for continued progress. Following his principle means acting as if we are discovering causes. But we aren’t: as Einstein himself was well aware of, our theories are ‘free inventions of the human intellect’. They are our answers to our questions. We can call them laws, if we are particularly pleased with how well they work. But they are our laws – not the great laws of nature.
This doesn’t mean that we make them up. We write explanations, but we read facts. We may even decide which facts to read but, ultimately, we submit to them. Explanations are not arbitrary inventions. They are based on the ultimate arbiter: evidence. Evidence is what there is – not phenomena appearing in a timeless universe, but events coming in and out of existence, along time.
Inverting Weyl: The objective world simply happens, it is not.