"evidence they've gathered adds up to a sufficiently high probability for P"

Perhaps I should ask what you mean by "evidence"? By evidence do you mean examples of an event happening that corroborates a particular theory that someone holds ?

So if 1. you have an expectation of something happening, and 2. that something happens,

then you are saying that the event is evidence in favor of the theory. And if the event happens even more when you expect it to then

1. it is even more evidence for the theory, and this increased probability is calculated by using a Bayesian rule to update your increased expectation of the likelihood of the truth of your theory?

Have I stated your argument correctly?

"For example I don't know how something that is true cannot ever be justified (how else do you know it's true!"

You can't know that something is true. We are fallible. And our best theories are often wrong. We gain knowledge by arguing with each other and trying to point out logical contradictions in our explanations. Experiments can help us to show that competing explanations are wrong (or that ours is!) .

Induction as a scientific methodology has been known (since Hume) to be impossible. Happy to discuss this further if you like. I will certainly read the articles you suggest. Please consider reading David Deutsch's, The Fabric of Realtiy. He (better than Hume in my estimation) shows the 'complete irrationality of induction, but I am happy to discuss, if you are interested.

There is no serious quantum physicist who would deny that it is possible to prepare a superposition of states in which a needle penetrates the skin to obtain a blood sugar measurement or does not. This situation could be created, perhaps by briefly freezing a small component of blood and skin on a live person. When this situation predictably resolves into a situation in which the measuring apparatus reads out the result of a blood sugar measurement, though the needle is seen to never penetrate the skin, where was the measurement made?

Where was the bloody needle? Where was the measuring apparatus on which the measurement was made. Where was the arm from which the blood was taken!

Those who do not understand the existence of the multiverse need to provide answers to these simple questions. If the arm is not real in a different universe in which the needle actually went in, how was blood drawn from it and a result reported?

If someone seriously doubts that this scenario can and will be created in the future, which law of physics says that we cannot create this superposition? Which law of physics do you plan to change, to prevent this result, though it has not failed any experiment?

Remarkably, even most of those who deny the existence of the multiverse do not deny that such a blood sugar result could be obtained. This means that virtually all physicists, including those who support Bohm, transactional perspective, Copenhagen, etc., agree that we will be able to obtain a blood sugar result from a needle that never penetrated the arm.

To them I ask again. Where is the arm from which the blood was drawn? Is your hypothesis really that it was drawn in the world of possibility? If so then the map that you call the world of possibility has every component of a real world, including the blood! When the map is as detailed in every respect as the territory, it is the territory. Right?

Although I also think Psi is bogus, my belief has nothing to do with the fact that previous claims of psi have been bogus. Evidence can never justify a theory, any more than finding 10 white swans in a row proves that there are no black swans! Believing that psi is false because of evidence that psi has been false in the past is the logical fallacy of inductivism. Most rational people do not believe in Psi because it has no logical theoretical/scientific basis and because it does not explain things well.

Much of this type of argument strikes me as nonsense. Something that is true can not be justified. One can (and should) argue that something is true. But argument is not justification. If the argument explains something well, then one should believe it, if it is the best theory available.

But evidence can never support any argument. It merely corroborates it. The reason that you believe a coin is fair is not ultimately because the results of an experiment convince you. It would be easy to set up an algorithm that causes the first 3000 examples of a computer simulated coin-flip to have the correct number of heads or tails to make the uninformed believe that the simulated coin flip is fair. But the next 10,000 could yield very different results, just by using an easy-to-create mathematical algorithm. No p-value can be assigned even after 3000 computer simulations of a coin flip. The data never tell a story (to quote someone on another site).

The reason we rationally believe the results of experiment when we flip the coin, but not when we see an apparent computer simulation of a coin flip is: In the case of the actual coin we already have explanations of the effects of gravity on two-sided metal objects, well before we have any data about coin flips. The same is not true about the computer simulation of the coin flip, unless we see the program ahead of time.

It is the theory about the effects of gravity on two-sided metal objects (with a particular pattern of metal distribution) that we try to evaluate when we flip coins. The data never tell us a story about whether the coin is fair. We first have a theory about the coin and its properties and then we utilize the experiment (the coin flip) to try to falsify our notion that the coin is fair if the coin looks balanced. Or, we falsify the notion that the coin is not fair, if our initial theory is that the coin does not look balanced. Examples of a phenomena do not increase the probability of it being true.

The reason we may believe that a coin could be fair is that we first evaluate the structure of the material, note that it seems to have a structure that would promote fairness given standard human flips of coins. Only then do we test it. But it is our rational understanding of the properties of the coin and expectations about the environment which make the coin flip reasonable. The results of any test tell you nothing (logically, nothing at all) about the fairness of a coin unless you first have a theory and an explanation about why the coin should or should not be considered fair.

The reason we do not believe in psi is that it does not explain anything, violates multiple known laws of physics, yet creates no alternative scientific structure that allows us to understand and predict events in our world.

He does not mean "lacking unnecessary details". For example the statements "Everyone just acts in his own interest" or "Everyone is really an altruist" are simple and lack unnecessary details, explain quite a lot, and are consistent with Occam's razor. But by Deutsch's criteria they are bad explanation because they are too easy to vary. For example, someone who believes in the self-interest theory could say, "John gave to charity because he would have felt guilty otherwise. So he really was selfish" .

We see that it is easy to change the theory that everyone is selfish to accomodate the case of someone who seems altruistic.

Or someone who believes in the altruist theory could say about John murdering Harry, "Well then, Harry must have been very unhappy."

The altruist theory and the selfishness theory are simple and explanatory in their own way, but too easy to vary. Similarly the idea that that sexism, feminism, capitalism, communism, parental coercion, environmental disregard, etc. cause unhappiness or mental illness or some other broad conclusion are equally meaningless. These explanations are bad because they can be varied to explain ANYTHING.

In contrast, theories that are difficult to vary go out on a limb. They are bold conjectures that explain a lot but even one small counterexample easily invalidates the whole thing. A good theory can not easily be changed to "take into account" the aberration. For example, Einstein's theory of gravitation is a good explanation because it explains a lot, it makes counterintuitive predictions, and even one repeatable counterexample invalidates the whole thing. It can't be easily changed to accommodate something else without invalidating everything else about it.

Theories that are hard to vary remain constant over time. They are more true and therefore more timeless. Invariable theories possess more verisimilitude ("truth-likeness" to use Popper's term).

Like the very best possible theory, truth also cannot be varied. It is completely timeless. It was, is, and always will be true, without any change. That is Deutsch's point.