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Causation and the Free Will Debate

Michael Egnor


In the debate over free will, free will deniers invariably offer three arguments against the reality of libertarian free will. They argue that nature is deterministic, that the mind is wholly a material phenomenon, and that “causation” is an incoherent concept if we accept indeterminism and free will.

The error in their first two arguments — that nature is deterministic and that the mind is wholly material — is obvious, as I have pointed out. Now I’ll address their third critique of free will — that “causation” is incoherent if indeterminism and free will are true.

Free will deniers argue that physical events in nature — for example, my choice to raise my arm — must have causally sufficient material antecedents. They assert that free will, as a kind of causation outside of material cause and effect, is not possible and is not even a coherent concept. Any meaningful understanding of causation in the material world, they argue, precludes free will, which is an immaterial cause, particularly if the world is indeterministic and probabilistic.

I point out that the fundamental indeterminism and probabilistic functions inherent to quantum mechanics are established science, and any genuine understanding of causation — whether it entails free will or denies it — must accommodate this science.

Free will in an indeterminate system governed by probability is entirely compatible with causation in the natural world. Consider the analogy of playing a lottery.

If I play a lottery, the outcome is based on probability. The likelihood that I will win is very low, but the probability that some person will win is quite high. The outcome of the lottery is, in other words, indeterministic and is described by probability functions, which is in fact what makes it a lottery. It wouldn’t be a lottery if the outcome were predetermined. In this way, the lottery is analogous to natural events at the quantum level, in which nature exists as an ensemble of wave functions of possible states until a measurement collapses the wave functions into an actual state. A lottery is an ensemble of possible outcomes described by probability functions until a drawing collapses the possible outcomes into an actual outcome.

Yet each step in the quantum mechanical system and in the lottery is caused in a perfectly natural way. The collapse of the wave function is caused by the original state of the system and by the measurement of the system. The outcome of the lottery is caused by the original state of the lottery system (the legislators who created the lottery and the administrators who run the lottery and by the actions of the people who buy lottery tickets) and by the selection of the lottery number, which causes the “collapse” of the probability function of the lottery to an actual winner.

In fact, the lottery analogy can be extended to any activity in life (life is, after all, a gamble). Taking a trip in a plane, driving to work, meeting a friend all entail probabilistic and indeterminate events (the possibility of an airport delay or a traffic jam or cancellation of the meeting, etc.), but that doesn’t mean that the outcomes of the indeterminate probabilistic system aren’t caused. Taking a trip or going to work or meeting a friend are all caused by ordinary causes in the natural world.

The fact that a system is probabilistic and indeterminate is entirely consistent with the fact that the system is caused. Causation by mental agency (the will) is no more problematic, in a logical sense, than causation by physical agency (a photon). To those who claim that we do not understand mental causation, I point out that we do not understand physical causation either (as Hume observed).

There is no inherent logical problem with free will and causation in an indeterministic system governed by probability functions (i.e. quantum mechanics) in the natural world.

Photo credit: © Yong Hian Lim —

Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.