Science and free will

The question if living beings, in particular those of our own species, possess “free will”, and how it works if it exists, has recently become fashionable again. The new idea that brought the topic back into discussion was that our sense of free will might just be an illusion. According to this idea, we would be machines whose fate is entirely determined by the laws of physics (which might themselves be deterministic or not), even though we perceive ourselves as actors who pursue goals and take decisions that are not even in principle predictable by a physical analysis of our bodies, no matter at what level of detail.

The topic itself is an old one, perhaps as old as humanity. I won’t go into its philosophical and religious aspects, but limit myself to the scientist’s point of view: is free will compatible with scientific descriptions of our world? Perhaps even necessary for such descriptions? Or, on the contrary, in contradiction to the scientific approach? Can the scientific method be used to understand free will or show that it’s a useless concept from the past?

What prompted me to write this post is a recent article by Anthony Cashmore in PNAS. In summary, Cashmore says that the majority of scientists do not believe in the existence of free will any more, and that society should draw conclusions from this, in particular concerning the judicial system, whose concept of responsibility for one’s acts is based on a view of free will that the author no longer considers defendable. But don’t take my word for it, read the article yourself. It’s well written and covers many interesting points.

First of all, let me say that I don’t agree at all with Cashmore’s view that the judicial system should be reformed based on the prevailing view of today’s scientists. I do believe that a modern society should take into account scientific findings, i.e. scientific hypotheses that have withstood a number of attempts at falsification. But mere beliefs of a small subpopulation, even if they are scientists, are not sufficient to justify a radical change of anything. As I will explain below, the question “do human beings possess free will” does not even deserve the label “scientific hypothesis” at this moment, because we have no idea of how we could answer it based on observation and experiment. We cannot claim either to be able to fully understand human behavior in terms of the laws of physics, which would allow us to call free will an unnecessary concept and invoke Occam’s razor to get rid of it. Therefore, at this time, the existence of free will remains the subject of beliefs and scientists’ beliefs are worth no more than anyone else’s.

There is also a peculiar circularity to any argument about what “should” be done as a consequence of the non-existence of free will: if that hypothesis is true, nobody can decide anything! If humans have no free will, then societies don’t have it either, and our judicial system is just as much a consequence of the laws of nature as my perceived decision to take coffee rather than tea for breakfast this morning.

Back to the main topic of this post: the relation between science and free will. It starts with the observation of a clear conflict. Science is about identifying regularities in the world that surrounds us, which permit the construction of detailed and testable theories. The first scientific theories were all about deterministic phenomena: given the initial state of some well-defined physical system (think of a clockwork, for example), the state of the system at any time in the future can be predicted with certainty. Later, stochastic phenomena entered the scientific world view. With stochasticity, the detailed behavior of a system is no longer predictable, but certain average properties still are. For example, we can predict how the temperature and pressure of water will change when we heat it, even though we cannot predict how each individual molecule will move. It is still a subject of debate whether stochastic elements exist in the fundamental laws of nature (quantum physics being the most popular candidate), or if they are merely a way of describing complex systems whose state we cannot analyze in detail due to insufficient resources. But scientists agree that a scientific theory may contain two forms of causality: determinism and stochasticity.

Free will, if it exists, would have to be added as a third form of causality. But it is hard to see how this could be done. The scientific method is based on identifying conditions from which exact predictions can be made. The decisions of an agent that possesses free will are by definition unpredictable, and therefore any theory about a system containing such an agent would be impossible to verify. Therefore the scientific method as we know it today cannot possibly take into consideration the existence of free will. Obviously this makes it impossible to examine the existence of free will as a scientific hypothesis. It also means that a hard-core scientist, who considers the scientific method as the only way to establish truth, has to deny the existence of free will, or else accept that some important aspects of our universe are forever inaccessible to scientific investigation.

However, there is another aspect to the relation between science and free will, which I haven’t seen discussed yet anywhere: the existence of free will is in fact a requirement for the scientific method! Not as part of a system under scientific scrutiny, but as part of the scientist who runs an investigation. Testing a scientific hypothesis requires at the very least observing a specific phenomenon, but in most cases also preparing a well-defined initial state for some system that will then become the subject of observation. A scientist decides to create an experimental setup to verify some hypothesis. If the scientist were just a complex machine whose behavior is governed by the very same laws that he believes to be studying, then his carefully thought-out experiment is nothing but a particularly probable outcome of the laws of nature. We could still draw conclusions from observing it, of course, but these observations then only provide anecdotical evidence that is no more relevant than what we get from passively watching things happen around us.

In summary, our current scientific method supposes the existence of free will as an attribute of scientists, but also its absence from any system subjected to scientific scrutiny. This poses limits to what scientific investigation can yield when applied to humans.

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3 Comments on “Science and free will”

  1. khinsen Says:

    In the July 13 issue of PNAS, there is a letter by Henrik Anckarsäter commenting the article by Anthony Cashmore ( and a reply by Cashmore ( Unfortunately neither text is in free access. Anckarsäter summarizes the biological and environmental constraints on human decisions that have been identified by scientific studies, but points out that they don’t explain 100% and that science therefore cannot claim to understand human behaviour. Cashmore replies that the absence of a full scientific explanation is not a justification for maintaining an unscientific belief in free will which just doesn’t fit in with our scientific understanding of the world.

    Both sides agree that science can’t fully explain human behaviour at the moment, but disagree about the conclusions that should be drawn. Personally, I’d prefer not to draw any conclusion at all.

  2. denis Says:

    “if that hypothesis is true, nobody can decide anything!”:
    I laughed, then winced. It seems that politicians are taking
    longer and longer to make real decisions, at least in national / international politics; for example, no new debt after 2020.
    Do political decision-makers have less free will than scientists, or perhaps *want* less free will ?

  3. […] sujet de l’article qui a tout déclenché, Konrad Hinsen nous explique dans un billet que la question du « free will » ne peut être posée en des termes […]

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