It is no great intellectual feat to create a model of the universe in which every event is predicated on every event which occurred before, with an argument of "Why not?"
It is the ultimate untestable hypothesis and ultimately, it doesn't matter. Like a lot of philosophical discussions which question the nature of existence, we aren't going to change anything about the way we live, no matter how convincing the argument the argument that we can't change anything, even if we wanted to.
It’s true you can’t test these claims. They‘re unempircal. I suppose that’s why a lot of scientists get exasparated with philosophers.
Suppose, again, ”D” stands for ”John puts on a blue shirt.” We observe antecedents A,B,C, and note that we get D.
The hard determinist wants to say that given A,B,C, we MUST get D — that John MUST put on a blue shirt.
The compatibilist merely says, much more parsimoniously, that given A,B,C, we WILL get D — John WILL put on a blue shirt, but he does not HAVE TO do that.
How are we to decide who is right? We can’t. The notions are underdetermined by the available data.
However, the compatibilist has logic on his side — he notes that contingent behaviors (“puts on blue shirt”) can never be necessarily true (triangle has three sides).
Regardless, if we were somehow able to “back up” the universe, and replay it again and again, the compatibilist has no problem in saying that the result will always be the same — given A,B, and C, we will get D. He will simply continue to deny, appropriately so, that D is a
necessary outcome of A,B,C.
We deal all the time in counterfactuals, so effortlessly that we hardly notice them. And yet I think counterfactuals are the key to understanding the compatibilist account.
I may say this morning, “I’m skipping breakfast, because last night I had a big dinner.” So last night’s big dinner
determines my choice this morning to skip breakfast.
But I might also put it this way: “I’m skipping breakfast, because last night I had a big dinner. If I had NOT had a big dinner last night, I’d eat breakfast this morning.”
So there is a possible world at which I eat breakfast, but at that world the antecedents are counterfactally different. This is called a possible nonactual world.
Antecedents A,B,and C don’t FORCE John to put on a blue shirt, they give him a REASON to do so, just like my having a big dinner last night gives me a REASON to skip breakfast this morning. Different antecedents, different reasons, and then different outcomes.