Posted by: dacalu | 9 August 2012


I’ve been reading up on questions of determinism for a class I’ve been teaching and wanted to set down my basic understanding of how we use the word in academia.  Notably, it means different things in Physics and Philosophy.

“Free Will” has had a number of meanings historically.  I usually use the word “agency,” meaning that whatever choice was made, you could have chosen something else.  This idea means you are making a statement about something that didn’t happen.  The philosophy jargon for this is “the contrapositive.”

For the record, I believe we have agency and will, but that our choices are not completely free.  So I’m saying agency and constrained will.

I like to differentiate between physical and philosophical determinism.

In physics, we say a system is deterministic if the future state of the system can be predicted without error from the present state of the system.  The dominant alternative is stochasticity – when we can predict a range of outcomes and their probabilities but cannot specify exactly what will happen.  One might also be a (physical) prediction skeptic and maintain that no confident knowledge of future events is possible.

In philosophy, we say the universe is deterministic if humans have no agency.  Historically, this may be because the Fates control our destiny, because God predestines our choices, because the stars exercise influence over our actions (like a clockwork mechanism), or because our biology, chemistry, or behavioral conditioning fully constrain our interactions.  Alternatively, we can say humans have the ability to change the course of events through causal agency. This is generically called “Free Will,” though that term has also been used to refer specifically to the idea that we might choose anything – to fly for instance, even if the choosing has no physical outcome.

The most common positions are –

Physical Determinism with Philosophical Determinism:  No agency, complete prediction.
Physical (Causal) Skepticism with Free Will:  We cannot predict the future because humans will affect that future and we don’t know what they will do.

Other positions, however, are possible, logically consistent, and historically relevant:

Physical Stochasticity with Philosophical Determinism: We cannot make predictions in light of quantum indeterminacy and chaotic interactions; human choices, however, are fully (if probabilistically) controlled by physical factors. [Science is going in this direction.]

Physical Stochasticity with Free Will:  Will is related to quantum indeterminacy.  This seems to be really popular among the religion and science crowd.  (I find it usually comes from misunderstanding quantum mechanics, but see Bob Russell).  Alternatively you could simply hold to both doctrines without saying they are related.  (I flirt with this position.)

Physical Determinism with Free Will:  Souls made their choice in eternity in the moment of creation, and everything else falls out automatically based on that one choice.  Alternatively, there is a school called “compatibalism” (dominant in modern analytic philosophy) that argues we have free will as part of a deterministic causal chain.  (Honestly, I don’t really get this.)

Physical (Causal) Skepticism with Philosophical Determinism:  It’s possible we have neither free will nor any ability to predict the outcome of events.

Therefore, physical determinism doesn’t mean you have to reject Free Will, or vice versa.




  1. […] I’m not messing with stochasticity here.  If you want to know more about that, see here.   The chain of events is determinate and unbranching because nothing other than physical […]

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