In 1644, Descartes formulated one of the basic elements of all modern science. In his Principes de la philosophie, he fixed what once for all would be the dividing line between theology and science. With Descartes’ analysis in mind, one would never have to mix the concepts, lest one produce bad science.

Before that, Galileo pointed the way to mathematics as the only language with sufficient clarity and precision to be able to handle all of nature’s phenomena. Galileo had, in addition, demonstrated the importance of humbly and sensitively questioning nature itself through rigorous and empirical experiments in which dreams and preconceived notions are replaced by indisputable facts. But the conclusion Descartes came to was, in many ways, even more profound and revolutionary, which was that scientists must never look for reasons based on future purposes. No explanation of natural phenomena should be based in the existence of a predetermined and final purpose. With this, the paths of natural science and theology were parted, finally and irrevocably.

This was, of course, a decisive break with the ‘old’ science, designed by Aristotle, where the search for the purpose was of central importance. Aristotle defined four types of causes: the material, the formal, the effective and also causes based on purpose. It is not easy to translate Aristotle’s characterisation of modern language, but broadly it is about the following.

The material cause corresponds to the matter that makes up what we are interested in, while the formal cause is the form that the matter takes. These first two types of cause are thus just about getting the actors in place before the events you then want to describe. The effective cause, however, is that which we in modern language would call causality – that is, cause and effect – and describes how one thing leads to another.

To illustrate the reasoning, it is perhaps best to use Aristotle’s own example, in which he reflects on why a particular statue came about. He notes that the material cause is in the form of the bronze that makes up the statue; the formal cause is who the statue portrays; and the effective cause is the sculptor who created it. But something is missing, of course: we must also understand the purpose for which the statue was created, which is the fourth of Aristotle’s causes. Without it, we can hardly say that we fully understand why the statue exists. Maybe the sculptor wanted to honour a king or a loved one? Maybe they wanted to be a famous sculptor or just make a buck? Or maybe it was simply for the joy of creation?

Aristotle states that ”a scientist must study all the four causes,” but he emphasises the cause of purpose as being of particular importance. He sees no fundamental difference between the things he can find in nature and those created by human hand. He argues, not least, that there must be a purpose behind how well an animal’s body parts interact. Of course, he admits, if the rain destroys crops, it is a coincidence unrelated to any purpose. But that chance might be the cause of life’s adaptation he dismisses as absurd. Empedocles’ belief – an early forerunner of Darwin’s theory of natural selection – that it was the beings that happened to be assembled correctly who survived, and that this would provide a sufficient explanation, he dismisses as manifestly unreasonable.

In the modern science that followed Descartes, we content ourselves with the first three of Aristotle’s causes. What is the reason an apple falls from a branch? The material cause is the matter that builds up the soil, and the apple tree. The formal cause is the form the matter takes. The effective cause is the force of gravity that overcomes the other forces in the apple’s stem, which causes it to drop. But to what end? None. There are no objectives or intentions of nature. No final causes. The apple does not fall for Newton to see and understand.

This is precisely among the most important, but perhaps the most difficult, that scientists must speak to the public interest. There is, of course, something so deeply human in looking for purpose. To ask the question why? We have an urge to create stories that give us an opportunity to understand and manage our own lives as well as the surrounding world. The good story requires a purpose, an intention, a meaning. As humans, we are constantly in search of this sense of meaning. This is an evolutionarily conditioned suspicion akin to the prudence and caution of dark nights where danger may lurk. We imagine how a wild animal or a robber could hide in the bush. A shadow of a tree or rock, we interpret as a living being with evil intentions. Those who do not have enough imagination do not survive. It is always best to err on the safe side.

We smile at the child who draws eyes and a mouth on things that we adults do not even know are dead. The sun is happy until the angry cloud slides across and makes her – for is the sun not a she? – sad. Cars and buses, trains and planes, all become creatures with intent and emotion in the fairytale world. One might think that this childish view to the world has faded away, and that we as adults no longer need these metaphors for a proper understanding of nature and environment dynamics. But such stories also work on those who are fully-grown. Their educational value is unbeatable, and I use it myself when I describe something in physics or science in general. I am somewhat aware that I actually do violence to what I want to convey, but I am convinced that it’s for a good cause.

Maybe I sometimes I might need to explain that the particles of particle physics can be divided into fermions and bosons? For fermions there are limits on how well you can pack them together, while the corresponding feature is absent in bosons. Electrons are fermions and that is precisely why atoms have the properties they have and why chemistry with its periodic system works at all. You could say that fermions are ‘unsociable’ and bosons are ‘gregarious’. This humanising I sometimes amplify further by drawing sad and happy faces on the various particles. It works equally well if the audience is made up of fifth graders or members of the Royal Academy of Sciences.

But sometimes it can go wrong and the educational ambitions are lost. A good story can take precedence and violate the facts that you wish to convey – as with the stories it’s so tempting to tell about evolution. It’s so easy to be lazy (but understandable) and claim that the gazelle through evolution has become swift to enable it to outrun the lion; that evolution intentionally creates good and useful characteristics and contributes to the development of objectively better and better characteristics.

But it is, of course, not so. The biological and genetic world lacks any intention. Everything is just a mindless consequence of genetic chance and merciless selection. (Not even here can I insert such a value-laden and misguided word as merciless.) This description of evolution, whose main point is precisely that it has no purpose, thus becomes an ironic and paradoxical negation in and of itself. Empedocles was right and Aristotle wrong.

And how is it in physics, which claims to be the most objective, beyond human prejudice, and the noblest of sciences? What about the quest for beauty and simplicity that is so often stressed? As long as it is about a proven methodology that experience leads to good results, it is unproblematic. But when it starts to be about a preconception of how nature has to be established, it becomes something else. Suddenly, one is thrown back into an ancient world where the very object of the world that Descartes wants to do away with becomes legitimate. Concepts like goals and purposes, including simplicity and beauty, presupposes a subject and/or an observer. Who decided the purpose? Who determines whether it is beautiful?

Physics will no doubt do just fine without the fourth cause. However, one can marvel at how incredibly well set up our universe is for us, and for life to exist. But instead of being, in this sense, a benevolent purpose, it is enough to imagine that the universe is simply much greater than we have so far been able to grasp; that it is large enough to accommodate both what from our perspective is good and what is evil. We have, of course, happened to be where the conditions, by chance, are sufficiently favourable to us, but beyond the horizon are new and alien worlds. Perhaps our universe is just a small island in an unimaginably large and rich multiverse?

This is certainly the sort of universe to which string theory – modern physics’ attempt to create a theory of everything – seems to lead. At first, the work was guided by the hope that in these equations could be found the only possible answer, and that we could get an explanation of our universe’s characteristics in the form of unique and exquisitely beautiful solutions to string theory’s equations. Everything would be fully defined by mathematical necessity, that our universe was created to realise the purpose and principle of perfect beauty. But instead, the theory in recent years has demonstrated the exact opposite, and the possible worlds’ riches seem far greater than anyone had imagined, feared or hoped for. In this way, Aristotle’s fourth cause is removed from physics and cosmology in the same way as it long ago lost its role in biology.

Science is, according to Descartes, by definition, about the part of existence where there is no purpose and ultimate cause. This kind of external world is in stark contrast to the subjective human realm where we all, scientists or not, live our lives. Aim and purpose are obvious factors in everyday life. We perform constant actions well aware of the motives and we interpret the corresponding intentions of our fellow human beings. The entire man-made world is full of meaning. Aristotle’s analysis of the causes of the sculpture of bronze feels, in this context, quite right and proper.

One must also distinguish between science as a phenomenon and the individual scientist’s personal motivations and motives. It’s not just Newton – only two years old the year Descartes formulated his principle – who, much later, continued to mix these different worlds. But really, this is not so important – it is results that count. In Descartes’ sense, objective and sound science may well be created by people whose worldview and motivation is in no way in harmony with the exalted ideal. The inspiration that points one in the right direction, and which provides a useful and fruitful solution, may come from an unexpected source. But the conclusion remains the same. Descartes was right. In nature, we never need to consider the fourth cause’s purpose. It is only in the subjective human realm where meaning and purpose seem inevitable realities.

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But what is really real? Is all this an illusion? Is this meaning we see only a construction? This is the great question that so much of the debate between the religious, agnostics and atheists is really about. Can the fourth cause always be reduced to the other three? How far does Descartes’ analysis go?

The question can, of course, be formulated from both a theoretical and a practical starting point. Even if the final causes in a fundamental sense are entirely missing, it may be impossible for us to take a perspective where this becomes apparent. It is, of course, paradoxical that we can ask ourselves outside of ourselves and reduce our own drive and quest for purpose to something that lacks these traits. We can have a conviction that the fact of the matter is one way or another, but this is not the same as proving our position with scientific clarity. Perhaps a different and higher extraterrestrial intelligence, developed on another planet, has the ability to analyse to the smallest detail in a way that our more limited brain cannot? And thus understand how it is all just about the neurons’ electrical impulses, or something else of which we had no knowledge? A way to explain to us inferior creatures in a comprehensible and convincing way, however, does not exist. We do not have, almost by definition, the ability to comprehend such things. But when these aliens then think about themselves and try to fully understand their own lack of purpose, they find themselves in the same helpless position as us.

And is this not perhaps the most crucial insight? That, irrespective of how astutely we think we can reason about the ultimate questions, there is no reason to really take things so very seriously? Some things we can say about heavenly movements, a bugs’ life, and atomic structures, but most aspects of our own humanity, and especially when Aristotle’s first three causes need to be completed by the confusing fourth, must remain preliminary and uncertain.

There are, after all, a number of basic facts to consider when dealing with these concerns. The human brain that we expect to be able to comprehend the universe weighs just over a kilo and contains around one hundred billion nerve cells. But this is no more cells than the galaxies in the visible universe. Our Milky Way is but one of these galaxies, which in turn contains a hundred billion stars, each of which weighs about a thousand billion billion billion kilos. This gigantic cosmos, with its history dating back nearly 14 billion years, is then to be grasped by a small gray lump, which is itself a part of that which it tries to comprehend?

Keep this in mind every time a representative of a religious philosophy, or a scientist, tries to convince you that he or she knows why the universe exists. Maybe they know less than they think.