If a tree falls in the forest, and no one is around to hear it, does it make a sound? Alice says yes, because it would produce a pressure wave through the air. Bob says no, because it wouldn't be perceived by anyone.
Alice and Bob aren't disagreeing about what happens when a tree falls down, they're disagreeing about the meaning of the word "sound".
Do we have free will?
Well, what do you mean by "free will"?
One notion of free will is that it is the ability to make decisions that are neither deterministically caused, nor random. This concept is called libertarian free will (unrelated to the political party), and it doesn't make much sense to me. To start off with, what's the middle ground there? How can something be neither determined, nor random?
Further, a non-determined decision wouldn't resemble a decision at all. Consider this scenario: you're in a building when a fire alarm goes off. So, you make a decision to evacuate the building. The only reason to evacuate the building was because the alarm went off. If the alarm hadn't played a role in determining your decision, then you wouldn't have made it.
So, that means we don't have free will, right? Well, it certainly means we don't have libertarian free will. But when you say things like "Free will doesn't exist.", people tend to react by saying things like "That means we can't make decisions!".
Which isn't true. Things that are completely physically determined can still make decisions. Consider a simple thermostat. If the temperature is below 18°C turn the heater on, otherwise, turn the heater off. Admittedly, that's in the fuzzy area between "decision" and "reaction", but I don't think it's fundamentally different than an undeniable decision, just simpler. (Which leads to another question of definition. What exactly is a decision?)
The fact that people tend to react that way implies that the libertarian notion of free will isn't the concept most people mean by the term "free will". So, we should find another definition that more closely matches people's free will. Here's my suggestion - Free will is the ability to make very complicated decisions that are determined by a very large number of factors and is very difficult to predict in advance.
Under this definition, we obviously do have free will. This doesn't resolve the issue. I'm still not certain exactly what a decision is. But I think it's a good step forward that helps avoid common misconceptions.