what will happen, will happen
is true by definition (right?). But to me that also implies that as long as time is linear, the future is predetermined (and hence there is no free will). What's the flaw in this reasoning?
This is called the problem of future contingents, and the conclusion is called logical fatalism. Already Aristotle struggled with it in the famous example of tomorrow's sea battle. By the law of excluded middle the sea battle will happen, or it won't happen. Whichever the case one of them is true, and by the nature of truth if it is true tomorrow then it is already true today. What will happen will happen, so it appears predetermined "by logic alone".
Although most agree that something is wrong here, there is controversy as to what exactly is wrong. Intuitively, there is no determinate "what" in "what will happen, will happen" before it happens, but to encode this intuition into logic is tricky. There is some controversy even on what Aristotle's own solution is, but it is usually assumed that he rejects bivalence for future contingents. In other words, they are neither true nor false until they happen (or not). This is one of those cases where the classical logic is not adequate to formalizing our reasoning, and systems of temporal (time-dependent) logic were created to do it.
The statement is only true by definition if you assume that "what will happen" is defined. That is, you're assuming determinism to prove its own correctness.
The statement reads to me like circular logic. If we replace future tense "will" with present tense "is", there seems to be no question that this is so:
"What is occurring today is occurring today."
We're simply stating the obvious and haven't really implied anything.
The argument that your statement suggests lack of free will would probably say that using the future tense "will" changes the meaning from stating the obvious to predicting the future, but I don't see how or why this must be so. At its core, the statement is still circular in nature.
Furthermore, the statement itself implies more mystery than definition by usage of the word "What": an inherently variable term capable of referring to anything, more or less.
You can have free will without this meaning that you modify the flow of things. You can also consider that through free will we modify the world around us. An absolute determinism that encompasses actions of organisms is most probably wrong. But a determinism considering cause-effect relationships of matter seems right. We should also distinguish between the concept of destiny-fate and determinism. Determinism regards the situation of the world and its causal relationships, and destiny/fate usually refers to a person or group and presupposes a superior will. Free will is usually opposed to the concept of destiny.
How to combine freedom and determination? It can be done as one aspect of freedom is the ability to determine things at will. And how the chain of causes and effects is affected by this? It can be said that we are determined to act as further determination agents.
from wiki article free will:
Determinism suggests that only one course of events is possible, which is inconsistent with the existence of free will thus conceived. As far as we know, this problem was first suggested by Aristotle in the fourth century B.C.E., but it is still a major focus of philosophical debate. This view that conceives free will to be incompatible with determinism is called incompatibilism, and encompasses both metaphysical libertarianism, the claim that determinism is false and thus free will is at least possible, and hard determinism, the claim that determinism is true and thus free will is not possible. It also encompasses hard incompatibilism, which holds not only determinism but also its negation to be incompatible with free will, and thus free will to be impossible whatever the case may be regarding determinism. In contrast, compatibilists hold that free will is compatible with determinism. Some compatibilists even hold that determinism is necessary for free will, arguing that choice involves preference for one course of action over another, requiring a sense of how choices will turn out. Compatibilists thus consider the debate between libertarians and hard determinists over free will vs determinism a false dilemma. Different compatibilists offer very different definitions of what "free will" even means, and consequently find different types of constraints to be relevant to the issue. Classical compatiblists considered free will nothing more than freedom of action, considering one free of will simply if, had one counterfactually wanted to do otherwise, one could have done otherwise without physical impediment. Contemporary compatibilists instead identify free will as a psychological capacity, such as to direct one's behavior in a way responsive to reason. And there are still further different conceptions of free will, each with their own concerns, sharing only the common feature of not finding the possibility of determinism a threat to the possibility of free will.
The flaw in reasoning is to tease out more out of a sentence, than is put into it.
What will happen, will happen
Is at one level, and at its barest a mere tautology; at another level - fate, and therefore the fatalism you mention; but only in part, for fate can be destiny, and a mans fate may be his character - and his character written; Omar Khayyam wrote in his Rubiyat:
The moving finger writes, and having writ
Moves on; and not at all thy piety or wit
Shall lure it back to cancel a half a word
Nor all thy tears Wash out a word of it
Khayyam, is referring here to the honourable scribes (kiraman katibin) who record both deeds, good and bad, that a man may do or think to do; and also to time, it's ceaseless forward motion.
To, which his reply is:
I heard a voice within the tavern cry:
"Awake, my little ones, and fill the cup
Before life's liquor in its cup be dry"
That is exert thy will, and let being be more than just being.
The answer to your question hinges on two important areas of ambiguity. One area of ambiguity is what qualifies as free will. The other area is a problem with understanding the precise meaning of the sentence:
What will happen, will happen.
This seems like a truism, and it is if it "will happen" is translated to the "future."
The future is the future ( =tautology).
But the sentence can also have another meaning:
A second longer version with the same tautological meaning:
future events cannot be changed since "future" refers to what will happen, and there will be tautological fulfilment insofar as the events that happen will constitute the future.
But also consider this interpretation:
What will happen in the future is already necessitated and pre-determined.
Here, we are not saying future events will happen because the events in the future are definitionally the future events. Instead, we are saying that "what will happen" is pre-determined and thus will happen. In other words, we've changed the meaning of "will" here to mean something about necessity -- not linguistic necessity but logical or metaphysical necessity.
There might be other ones as well, but the point is that natural language is not always precise, and in this case, it creates a witty statement that means either nothing or a substantial philosophical claim.
Presumably, interpretations 1 and 2 are pretty trivial and have nothing to say about free will. The last interpretation, however, might say something about free will.
Free will is actually difficult to define. An initial consideration to keep in mind is that freedom should never be confused with randomness.
One common definition though disputed is the principle of alternative possibilities. This is the view that an action is free if and only if an agent could have done otherwise. A second definition of free will is that an agent is free if they choose their actions -- even if we could externally generate circumstances where they would choose the action.
Another related idea is that an action is free if it is autonomously chosen where autonomy incorporates a type of rationality. Some thinkers distinguish sharply between autonomous actions and free actions.
If future actions are fore-ordained or otherwise predetermined, one might still be free if one accepts a definition other than PAP for free action. If one accepts PAP, then it is a problem that future actions are determined and you would need to reject the third interpretation of the sentence. You may also possibly have some explaining to do with respect to the first and second interpretations in terms of how you understand the future.