Determinism and free will completely contradict each other and cannot be both true simultaneously.

In order to decide whether or not free will and determinism can put aside their differences and be friends, we have to figure out just what the heck they ARE. Every person who's started a similar topic has put forth their own definitions, and those definitions always fall apart. Why? Because they only APPROXIMATE what free will and determinism are. Determinism, by the definition he gave, is false right off the bat. There are things in nature that we can not compute because of the Heisenberg Uncertainty Principle. We do not live in a clockwork universe. The definition of free will doesn't fare any better: I don't know about you, but I've never been able to act in a way that I didn't act. What if we turned the clocks back and let the universe run again? Would it turn out the same as it is now? If A lead to B, then if we go back in time right before A, would B still follow? This is how many people frame the debate. You must ask yourself, though, what exactly does it mean to say "A would inevitably lead to B"? We can look at it from a physics standpoint or from a cognitive standpoint, and these two standpoints do not get along. From a physics standpoint, the Heisenberg Uncertainly Principle prevents just about anything from being inevitable. For most events that we're interested in, however, the HUP only presents an infinitesimally small chance that things might not have gone the way they did. This may not be true if you are interested in exactly which way a particle of radiation travels from a decaying atom, but unless you're a cat and your owner is named Schrï¿½dinger, you probably don't. In day to day life, we treat "infinitesimally small" as being "zero". Nobody will ever get out insurance in the case that the particles of their flower pot happen to spontaneously form into the shape of a working atomic bomb, even though physically speaking there is a non-zero probability of that happening. From a physics standpoint, saying "A would inevitably lead to B" means "the probability of A leading to B is so high, that it is not even worth considering the possibility that it wouldn't". From a cognitive standpoint, saying that "A could have lead to C instead of B" means that, in your mental framework of the world, it is consistent for A to have lead to C. You could, for example, say that "Tom could have taken the train instead of the bus". You could not say "Tom could have grown wings and flown instead of taking the bus", because that contradicts our model of how people should behave. From a cognitive standpoint, saying "A would inevitably lead to B" means that "It would contradict the nature of A to do anything except lead to B". Going back to physics, though, the difference between Tom taking the train or the bus depends on exactly how signals travel within his brain. The brain is largely a black box, but we can imagine that the HUP could allow signals to travel in different manners. Hence Tom could have taken the train or the bus. However, the HUP also allows for that possibility of Tom sprouting wings and flying to work. We can't even say which is more likely, as we only have a vague understanding of how the brain works. The peace between free will and determinism can be found in the distinction between the physics standpoint and the cognitive standpoint. If the actions of a human being are deterministic, then that means it is possible to predict, with near 100% certainty, what a given person will do in a given situation. From a physics standpoint, this is true. If you knew the exact state of the brain, you could work out all the equations, figure out what signals would go where, and find the most probable course of action the person would take. You may die of old age before you finished calculating, but theoretically you COULD. From a cognitive standpoint, however, we will never be able to have enough information, or enough time, to process all of the physical interactions going on in the brain during any decision. What's more, any instrument that could even TELL us that exact state of the brain would have to affect the brain itself, by the Heisenberg Uncertainly Principle. Therefore, we can NOT predict with near certainty what an arbitrary person would do in an arbitrary situation. In conclusion, the difference between free will and determinism is all in how you frame it.