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here’s a relationship between free will, and the ability to conceptualize a human doing different things. That is, to say that something is determined is to say that it is in its nature to respond to certain stimuli (correctly categorized) in a certain way (e.g. a billiard ball moves in a determined way when struck with a cue). However, individual human beings are conceptually capable of reacting in a multiplicity of ways given any particular stimulus.
The important thing here is that it is NOT VALID to propose “all physical interactions happening at a given moment” as a stimulus for a human being. This is an unacceptable reduction in what it is that the human mind is actually responding to. To make this point clearer, consider again the billiard ball: in order to predict its movement, we do NOT need to know the position and velocity of every single atom in the system. We just need a few variables, such as cue hardness, ball elasticity, friction coefficients, velocity, a couple angles, and so on. its trajectory is then calculable to a high degree of accuracy.
What is the human equivalent of these variables? The point is that the relevant reasons of the situation as understood by the human actor are the variables. The further point is that any particular human can conceivably act in a variety of ways all of which make sense within the conceptualization of the human-as-actor.
This is what it means to say that human beings are undetermined, or free, in their actions. The basic unit of stimulus to the human mind (the “reason”) does not produce deterministic response. This is in contrast to a billiard ball, which cannot be conceptualized behaving differently under the same limited stimuli. There are usually elements of arbitrariness or randomness relevant to how a human acts, but these factors are “rolled into” the choice, and a human acting freely can always give a valid account of why he acted in a particular way that does not have arbitrariness as a significant feature.