there's no such thing as free will

premise I: only matter exists (no God, soul, etc.)

everything exists in the physical, or in some way supervenes on the physical. that is to say, even abstract things like ideas and dreams have a specific and unique dependency on something physical. for instance, two individuals with the exact same brain state will have the exact same thoughts, memories, emotions, etc. (this relationship is called ‘supervenience’).

this premise (called ‘physicalism’) is not definitive, and even a strict but rational atheist would admit that there’s a non-zero chance that God or something else metaphysical exists. nevertheless, we can treat physicalism as a base truth until compelled to believe otherwise, given the burden of proof on metaphysical beliefs.  

conclusion I: the brain acts in a purely physical manner

implied by premise I. the brain is a physical system and, like all other physical systems, operates entirely according to physical principles, with no influence from metaphysical entities.

conclusion II: every event is either causal (deterministic) or random

given premise I, we can decompose all matter into a finite set of known characteristics: position, movement, atomic composition, etc. also given premise I, we know that this set of “all matter” includes everything that has, can, and will exist. from this, you can imagine a universal state function that predicts the incremental state of all matter (and therefore, everything) after each infinitesimal tick of time. we can define an "event" as the change in the physical state of matter across each tick.

every event must satisfy one or both of these two conditions:

1) the event is predictable, guided by known laws of the physical universe. for instance, when i throw a football, i can know with certainty where it will be in a second given enough information about its exact position, its velocity, the wind velocity, etc. and the laws of physics.

2) the event is random, such as quantum randomness like radioactive decay or photon behavior in the double-slit experiment. some scientists take this a step further and believe that synaptic transmission and ion channel activity in the brain may involve quantum randomness, potentially adding randomness to human behavior. while we can’t say with certainty this randomness exists (perhaps we simply haven’t figured out how to precisely model these events), we should reserve this possibility. 

these two cases are mutually exclusive and completely exhaustive - every event, every movement of any atom in the universe across any period of time, is either fully deterministic, fully random, or some combination of the two. as a result, every event is caused exclusively by predictable preceding events and/or some randomness. 

this argument is known as ‘causal determinism’ or ‘strict determinism’ (although sometimes these labels don’t include the randomness factor - the label that does is ‘causal closure’ or ‘physical closure’).

conclusion III: every action of the brain is predictable or random

apply conclusion II to conclusion I. given that the brain is purely physical, every event inside it is purely the predictable movement of atoms in a causal chain, imbued with some level of randomness. as a result, every action within the brain (every neuron firing, every release of hormones, etc.) is determined by prior states with some random influence.

an important implication of this conclusion is that there are only these two inputs (causal steps and randomness) that influence brain activity, without any room for a third category of “uncaused but willed” brain activity.

conclusion IV: there’s no room for an individual to have autonomous agency because every event is caused by prior states and/or randomness

a necessary conclusion following conclusion III. 

this one is the hardest to internalize - whether deterministic or indeterministic, the outcomes of brain processes are not the result of a self that can transcend physical law. The agent (the "self", or a conscious entity presumed to possess willful control over actions) is itself a product of prior physical events and cannot originate new causal chains ex nihilo (“out of nothing”). as a result, the self is simply a construction of previous states without any transcendent autonomous component. we lack the capacity to initiate actions independently of prior physical or random influences, to be the true originator of a causal chain.

another way of looking at this: at any given moment in time, before your body performs an action, your brain orders your body to perform this action by firing neurons accordingly. these neural events are triggered by perceptual stimuli (your environment) and your current brain state (memories, and other states in your brain, which were either developed over time, i.e., "nurture," or existed since birth, i.e., "nature"). these two factors, your environment and the state of your brain, along with some randomness, are the only inputs that result in each neural action. there’s no room for agency - when decomposing the causal chain of events before every action you perform, every atom’s movement can be mapped to some prior state and/or a random event, preventing the interjection of any autonomy in the causal process.

conclusion V: there’s no such thing as free will

implied by conclusion IV. agency is a necessary condition in achieving free will.