From a purely scientific perspective, the validity of a hypothesis is evaluated based on empirical evidence rather than a priori beliefs or assumptions. The scientific method involves formulating a hypothesis, making predictions based on that hypothesis, conducting experiments or observations to test those predictions, and then refining the hypothesis based on the results. This process may be repeated many times, with each cycle providing more evidence and making the hypothesis more robust.
In the case of the psychic phenomenon, the hypothesis that a person can consistently guess the number you're thinking of between 1 and 10 is testable. If the psychic gets it right significantly more often than would be expected by chance (about 10% of the time, if the numbers are chosen randomly), then that would be evidence in favor of the hypothesis.
However, it's important to be cautious about jumping to conclusions. Just because the psychic hypothesis fits the data does not automatically mean it's correct. There could be other explanations that also fit the data. Maybe the psychic is cheating, or perhaps there's a pattern to the way you choose numbers that the psychic has picked up on. In fact, these explanations are more consistent with our current scientific understanding than the notion of psychic abilities.
Can he do this with another person in another place? How do we know you are not a part of his team? There are lots of tricks like that done by so called "mentalists".
Now, regarding the a priori argument: The problem with this approach is that it's not based on empirical evidence. Instead, it's based on pre-existing beliefs or assumptions, which may or may not be valid. While it's true that we all have a priori beliefs that influence our interpretation of evidence (this is the basis of Bayesian inference), it's risky to rely too heavily on these beliefs when evaluating a hypothesis. Doing so may lead us to overlook important evidence, or make us too quick to accept a hypothesis that confirms our beliefs and too reluctant to accept one that challenges them.