[stackm]

It seems clear that there are two distinct possibilities when considering how the world and its inhabitants operate. Either

A) All actions are fully predetermined by the laws of physics, etc., and an omniscient being with full information could give you the complete state of the universe at any point in space-time; thus, free will does not exist

or

B) Humans, and potentially other beings, living or otherwise, have some degree of free will and volition, and thus have the power to affect the future state of the world to the extent that it cannot be predicted fully, even with full information.

Quantum mechanics and the like may provide us with a situation in which the state of the world is predetermined but with certain probabilities, but let's set that aside for the moment (I don't think it makes much difference in any case). Clearly, one of the two possibilities is the actual state of nature. If we are to assume that A is correct, then it seems as if our traditional standards of morality go out the window. After all, morality is about choices: murdering an innocent is presumed to be "wrong" because one could have had refrained from this action, yet chose not to. If a crazy , humongous serial killer grabs my arm and uses it to beat a little old lady to death, most people would not say that I did anything wrong: while it's technically true that I beat Grandma to her grave, I had absolutely no choice in the matter; the serial killer was entirely responsible for my actions, for better or for worse. It is difficult to see where morality can fit into a purely determinate world.

Thus, again consider the state of your own personal mentality if situation A is indeed correct. If you were to believe in choice B, you would be incorrect - but is there anything "wrong" with being incorrect here? After all, you have no say in the matter, and in fact you're not right or wrong at all, you're just carrying out the thought process that you must carry out, with no appeal to logic or reason. Your wrongness has no consequences since it has no alternative.

Let's say, however, that situation B is correct. Now, if you believe in situation A, you are again incorrect. However, this time there may be consequences; in a world where we are free to make decisions, maintaining an invalid belief may prove costly. Believing in determinism may lead to choices that are practically irrational (drawing to a the low end of a straight), as well as morally deplorable (socking the dealer when your straight hits and you lose to the high end).

Therefore, it appears as if determinism is in fact correct, it does you no good to "believe" in it, but if it is not, it may do you much harm. Therefore, you should do your best not to believe in (or at least not to act in accord with) determinism, as this decision seems to leave you with the highest possible EV. Note that this analysis is not analogous with the "better believe in God in case he actually exists" argument - in that case, there may be negative consequences to believing in a higher power if in fact there is none. Here, however, it's not even clear what it would mean to be a determinist if that were the reality, since your belief would have no meaning or importance (nor would any other action or happening).

In conclusion, it is the nut best belief to trust in free will, whether or not it actually exists. Make sense?