While he considered self-evident that everything has a cause, Laplace knew that causes themselves are not self-evident. Events do not come with their causes and effects attached. We are not demons: we see events, but not their ties to other events. If we want to see the ties, we need to discover them. As David Hume memorably put it:
I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation [of cause and effect] is not, in any instance, attained by reasoning a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other. (Enquiries Concerning Human Understanding, Section IV, Part I, p. 27).
We see the ball, our hand dropping it and the floor on which it lands. But we don’t see the ties between these events until we discover them through experience. By repeated experiment, we learn that, no matter how many times we drop it, the ball will always land on the floor, and it won’t land on the floor unless we drop it. It does so regularly, i.e. according to a rule. The ball is in free fall, but its fall is not free at all. Like us, Isaac Newton discovered the rule by experience – not a ball in his case but, famously, an apple. Unlike us, however, he realised that it was the same rule that forced planets to rotate around the sun, and called it the Law of Universal Gravitation.
Newton discovered the law but, to his eyes, the law was already there – he didn’t make it up. It was one of ‘the great laws of nature’, written in the grand book of the universe, not by trusted authorities but by nature itself for everyone to read, providing he knows the language in which the book is written. Galileo Galilei had expressed the same concept a few decades earlier, when, discussing the nature of comets with Orazio Grassi – a Jesuit astronomer writing under the pseudonym Lotario Sarsi – famously wrote:
Furthermore, I seem to detect in Sarsi the firm belief that in philosophizing one must rely upon the opinions of some famous author, so that if our mind does not marry the thinking of someone else, it remains altogether sterile and fruitless. Perhaps he thinks that philosophy is the creation of a man, a book like the Iliad or Orlando Furioso, in which the least important thing is whether what is written in them is true. Mr Sarsi, that is not the way it is. Philosophy is written in this all-encompassing book that is constantly open before our eyes, that is the universe; but it cannot be understood unless one first learns to understand the language and knows the characters in which it is written. It is written in mathematical language, and its characters are triangles, circles, and other geometrical figures; without these it is humanly impossible to understand a word of it, and one wanders around pointlessly in a dark labyrinth. (The Assayer, p. 183).
The belief that our description of the world coincides with the world itself goes back to ancient Greece, where logos meant ‘word’ – from legein: to say, speak and gather, collect – as well as ‘reason’, ‘logic’. What is spoken, gathered in the book of the universe is the world as it logically is. Later on, the Greek author of John, the fourth, ‘philosophical’ gospel, used the same term in the incipit: ‘In the beginning was the Word, and the Word was with God, and the Word was God’. The logos was Jesus: the link between mankind and the divine.
If what we say is what is, experience is just the unveiling of necessary laws. Balls and planets are bound to obey a law, and experience cannot but confirm it. Thanks to its mathematical formulation, the law allows us such wonders as precisely anticipating where the ball will land and predicting the exact time of tomorrow’s sunrise. We are sure about it, i.e. se-cure: free from the peril of the unknown.
But experience without peril is no experience at all. To experiment means being exposed to the possibility that the tie between the tested hypothesis and its conjoined evidence can fail. Failure can happen in two ways: False Positives – the ball does not land on the floor after I drop it – and False Negatives – the ball lands on the floor without me dropping it. If, after repeated experiment, I observe no failure, I conclude that the ball lands on the floor if and only if I drop it. This can be rephrased as: Dropping the ball is perfect evidence of it landing on the floor; or: Dropping the ball causes it to land on the floor, i.e. the ball lands on the floor because I drop it.
Since this unfailing regularity applies not only to our ball but to all objects (planets included), we call it a law of nature and name it gravity. Gravity is a satisfactory explanation or, as Spinoza and Laplace would say, a sufficient reason for why objects behave the way they do. An overwhelming amount of confirmative evidence proves that Newton’s law is true. But, as Hume pointed out, the law cannot be demonstrated on the grounds of pure reason. There is no a priori reason for which the ball must land on the floor and the earth rotate around the sun. The reason why we are certain about tomorrow’s sunrise is not that it is logically true, but that it has never failed to happen.
Like science is separating true from false, certainty is also a decision. Certus comes from cernere, which, like scire, means to distinguish, discriminate, discern. Laplace himself calculated the probability of tomorrow’s sunrise as (n+1)/(n+2), where n is the number of days in which the sun has risen so far. Hilariously (except to Young Earth Creationists), he assumed n=1,826,213, or 5,000 years, and concluded that ‘it is a bet of 1,826,214 to one that it will rise tomorrow’. To which, however, he hastened to add:
But the number is incomparably greater for him who, recognizing in the totality of phenomena the regulating principle of days and seasons, sees that nothing at the present moment can arrest the course of it. (A Philosophical Essay on Probabilities, p. 19).
What can be incomparably greater than 0.999999? It is BR=1: the faith that nothing can arrest the course of the Mécanique Céleste that Laplace had masterfully described in his five-volume oeuvre. His certainty in it was de jure as well as de facto. Sunrise was not only a sure bet: it was the demonstration of an inexorable principle, which was revealed by experience but in no way endangered by it.
If experience is merely reading from a book that has already been written, the only danger is misreading it – as, to his embarrassment, happened to Galilei in his exchange with Sarsi. The Assayer, written in 1623, was Galilei’s rejoinder to Sarsi’s Libra Astronomica ac Philosophica. Libra is a balance, on which Sarsi weighed different views about the origin of comets, three of which had appeared in 1618. Sarsi favoured the view of Tycho Brahe, whose cosmological system was approved by the Jesuits. Brahe thought comets were actual celestial bodies, rather than atmospheric phenomena due to sunlight shining on water vapour, which was Galilei’s view – ironically close to the traditional Aristotelian notion. Galilei opposed to the coarse Libra, on whose arms Sarsi had weighed a mixed bag of fanciful arguments, his refined Saggiatore, the ‘exquisite and fair’ balance used to weigh precious metals. Alas, it did not help: resembling what happens in much of today’s economics, Galilei reached mathematically precise but entirely wrong conclusions.
Galilei did not realise that we don’t just read the great book of the universe: we write it. What we call natural laws are local explanations that satisfy us and successfully stop us from asking further questions. But the reason we stop is not that there are no more questions to be asked. On the contrary, each answer begets new questions.
The more we explain, the more we ask. Our earlier ancestors were easily satisfied. As long as explanations came from a trusted source, they could be made of the weirdest concoctions of myths and legends – many still very popular. But reasonable explanations could be as satisfactory, and as wrong: if every celestial body rotates around it, and every object falls towards its core, the earth must be the centre of the universe. It is our childish urge to keep asking why that breeds new explanations for some of the questions that old explanations could not answer.
At the same time, accepting local explanations, and getting on with them without further questioning, is as important a prerequisite of our existence. Other animals get by without explanations. They know what happens if, not why it happens. We need to know why, but also decide what to believe. We do so individually, ultimately leaning upon soft evidence emanating from trusted sources. To believe is to hold dear, to love. Like credere in Latin, it comes from the heart. Each of us can believe anything. But mankind as a whole has nothing else to lean upon but itself. It is we who decide what is written in the book of the universe. Our explanations are verbal acts that we share through language and agree to accept. As Albert Einstein, who rewrote gravity into his theory of general relativity, put it:
Fundamental principles … are free inventions of the human intellect, which cannot be justified either by the nature of that intellect or in any other fashion a priori. (The Herbert Spencer lecture, in Ideas and Opinions, p. 272).
We are children who keep asking questions and grownups who keep inventing answers, with no idea of what the ultimate answer that ends all questions looks like.