Believe What Will Be: Pragmatism and Magic
King Arthur didn't always ask Merlin to foresee or intervene in what will be. For example, if Arthur had strong reasons to believe that his forces were stronger than the other army, he would not need Merlin's help or guidance. But when the outcome was uncertain, when there was no clear outcome—no evidence or equivocal evidence—and the stakes were great, Arthur turned to Merlin.
Merlin is a wizard. Wizards use magic. Magic is how we can trick ourselves or can be tricked into believing something. The belief could be false, as with stage magic (he didn't actually saw her in half), but this is a relatively uninteresting case. The more interesting cases are where the magic—the trick—contributes to and/or makes the belief true. Such as: who will win the battle that Arthur is asking about. In situations where the outcome is uncertain or the evidence is equivocal, Merlin might look for portents. Portents can tip the scales of belief. Perhaps Merlin sees an eagle carrying a snake. This means that the day can belong to Arthur if they fight valiantly. This will invigorate Arthur, who will likewise rouse his men. Let's fucking go and make it happen. Therefore, magic makes things happen.
This has some parallels in pragmatism. Charles Sanders Peirce considers such practically equivocal options in “How To Make Our Ideas Clear” (HTM). In HTM, Peirce says that belief precedes doubt and that doubt disturbs belief. Peirce implies in HTM that when the consideration that caused the doubt calls for no distinctive results, the doubts are (should be) defused and we should discard them and revert to the settled belief that preceded the doubt. Similar to Arthur's situation, Peirce considers cases where the evidence is lacking or equivocal.
Compared to Peirce in HTM, William James is proactive in “The Will to Believe” (WTB). James says: when thou dost encounter intellectually irresolvable options, thou art free and justified to believe what thou wilt. In fact, the belief can be a contributing factor in establishing its truth and can in some cases definitively make the belief true. In other words, if there is no disparity between the effects of two options (no evidence at all or equivocal evidence for example), then you are free to and justified in believing what you fancy and you should believe what you fancy.
I stated above that magic is a trick. This statement of course begs for the following question: then what is a trick? There is the trickster and the tricked and they need not be different, one can trick themselves. In a trick, authority is transferred to some other—you let them take the reins. Arthur transfers the authority to Merlin who will tell him what is true or tell him what to do. Just as your financial advisor (an expert) tells you (not an expert) that you should transfer most of your money out of tech stocks. You are transferring the authority to them. Not only that, but you are transferring the agency to them as well, because you will carry out their instructions. If Merlin says you will be destroyed if you do X, then Arthur doesn't do X.
Analogously, Merlin transfers the authority and source of this information, power, will, etc. to something bigger and beyond himself. Merlin has “the Sight” and can see “what will be”.1 Merlin can read portents. These visions and signs are outside Merlin. Merlin is tricked by the signs and Merlin tricks Arthur. Merlin may also sometimes interfere and meddle, and in this case Merlin is not tricked by some signs and is just exercising his own will when he tricks Arthur. But what actually made it happen? It's a good story, but is it true? This line of questioning is a metaphysical quagmire with no bottom. Leave well enough alone. It worked.
In pre-scientific ages people were reasonable too. They learned from their mistakes and generalized and predicted based on the past. They didn't go straight to wizards and magic for every question. They would go to the wizard when there was a true dilemma. They would use tangible evidence and reasons when they could, but when there was nothing tangible, they asked those who could break the deadlock. Thou art free and justified to believe what thou wilt and believing it can make it true, but you might have to trick yourself (or have yourself tricked).
For more on pragmatism and magic, check out The Journal of Sophistry Vol. 1: PragmaTism + Magic = PragmaGism: Magic Over Mechanism
See: Excalibur (1981, dir. John Boorman)