I'm really struggling to comprehend soft determinism/compatibilism. How can free will be compatible with determinism? Surely by definition, they both necessitate exclusivity to each other?

Well, if "determinism" is defined as "the opposite of free will" or "whatever takes away free will", then they are definitely incompatible, but in a really uninteresting way, one that could not generate so much philosophical debate. Sometimes, people think that's what determinism means (and some dictionaries define it that way), and of course, that makes it very hard to convince people that compatibilism is not crazy.

But compatibilism is not crazy. In fact, the majority of philosophers are compatibilists, and my work in experimental philosophy studying ordinary people's intuitions about free will suggests most folk are compatibilists too, once we make sure they understand what determinism means (not that majorities cannot believe crazy things, but if we are trying to figure out how to define "free will" and its relevance to our practices of moral and legal responsibility, then understanding the way people actually understand the issues is surely relevant). So, why isn't compatibilism crazy?

Because determinism does not entail some of the threatening ideas people often take it to imply. Determinism is the thesis that a complete description of a system (e.g., the universe) at one time and of all the laws that govern that system logically entail a complete description of that system at any later time. This thesis can be defined in terms of causation like this: every event is completely caused by prior events in accord with the laws of nature, such that, necessarily, given the earlier causes, the later event occurs. Assuming human decisions and actions are events, then determinism would entail that every human decision and action is completely caused by prior events in accord with the laws of nature.

But this does not mean that human decisions and actions are unimportant. Quite the contrary. They are part of what determines what happens in the future. So, the future depends on what we do, and determinism does not entail fatalism, the idea that certain future events will happen no matter what we do or try to do (e.g., it was fated that Oedipus would marry his mother no matter how he tried to fight that fate). Similarly, determinism does not entail that the past controls us or that nature predestines what we do or that we are manipulated in any way.

Just as the future depends on what we do, what we do depends on the past. But crucially, our decisions and actions can depend on the parts of the past that we want them to depend on, such as our deliberations, our conscious reasoning, our plans, our desires, etc. Nothing about determinism rules out our ability to envision different future possibilities (each of which is possible in the sense that whether it becomes actual depends on what we decide) and then making a decision based on our conscious considerations of the reasons for each of these alternatives. So, determinism does not entail what I call bypassing, the idea that our conscious deliberations, our desires, or our reasons play no role in what happens--that, instead, our actions are brought about by causal forces that bypass our conscious selves. (If we are manipulated by external agents or coerced by external forces or compelled by internal compulsions, we may not have free will, but these threats are all distinct from determinism--they are the threats compatibilists focus on).

My research suggests that when people think determinism entails bypassing--which it doesn't--that is when they take it to threaten free will. And when people do not interpret determinism to entail bypassing, but properly recognize that it allows that the future depend on our decisions and our decisions depend on our mental activity, then they think free will is compatible with determinism. Many people seem to associate determinism with a reductionistic view that suggests our conscious mental activity is bypassed by lower-level brain processes. Hence, the common misunderstanding of the relevance of neuroscientific discoveries to free will: "Don't blame me. My brain did it." But determinism does not entail this problematic reductionistic view.

Now, determinism does entail that our mental activity and our decisions are brought about by prior events, and those by prior events, ultimately going back to events before we were born, all in accord with the laws of nature (events, and laws, over which we had no control). So, there is a worry here, the worry that there are sufficient conditions for what we do, conditions over which we ultimately had no control. And this is where the philosophical debates concentrate their attention. I won't rehearse these debates here, but the compatibilist typically argues (a) that free will does not require the ability to do otherwise, holding fixed all prior events, in which case determinism is not a problem, (b) that free will does not require that we have "ultimate control" over the long-distant-past conditions that are sufficient for our actions, and (c) that the sort of free will incompatibilists say they want is incoherent (e.g., it doesn't make sense to think that one could make decisions based on reasons without there being a prior set of events which brought about those reasons, or to think that one can create oneself from scratch as "ultimate control" seems to require).

I won't rehearse those intricate, sometimes fascinating (sometimes frustrating) debates here. But I hope I have cut off some of the routes that lead people to think that compatibilism makes no sense, routes that go through misunderstandings about what determinism means. Once we cut off those routes, the threat of determinism is severely diminished (I think it is extinguished), and compatibilism does not look crazy at all.

(I have avoided the term "soft determinism", which is the claim that (a) determinism is true and (b) we have free will. Few compatibilists hold this view, because few are convinced determinism is true, given the most prominent interpretations of quantum physics. Most philosophers do not think that quantum indeterminism can help secure free will, which should make us wonder if the real issue is determinism or some other potential threat. Indeed, compatibilists need not be committed to the claim that (b) we have free will, since they are only committed to saying determinism does not rule out free will. Other potential discoveries, distinct from determinism, such as the irrelevance of consciousness in action, might be a threat to our free will.)

Here is a side note to your question. Soft determinism consists of two propositions: (1) the the thesis that determinism is true; (2) that it is compatible with freedom. Compatibilism on the other hand is merely (2). So soft determinism includes compatibilism, but there is more to it. I am a compatibilist but not a soft determinist (I am a compatibilist indeterminist), as I believe that there are some events that have no causes (denial of universal causation), and I also believe that the state of the universe plus the laws of nature do not determine the next state of the universe (determinism), and I also believe that some human actions are free. The only other compatibilist indeterminist I know of is David Lewis.

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