I argue that, for any falsifiable hypothesis, a prior probability of exactly 0 or 1 is irrational; to assign such a prior is to explicitly abandon one's capacity for reasoning in the face of evidence.

When faced with a new hypothesis, a rational agent should never assign a zero prior. Among other things, it leads to contradictions where two new hypotheses are encountered, both assigned a zero prior, but then it is discovered that, logically, at least one must be true, but the agent is prevented from ever getting away from their zero starting point credence. An agent that assigns a prior of exactly 0 or 1 is not exhibiting sound reasoning and will consequently make poor decisions.

Commonly, rational agents apply the principle of indifference to new hypotheses, assigning a prior of 0.5 to any new hypothesis.

However, as the agent first contemplates the new hypothesis, they may discover that existing, known evidence bears on the hypothesis, and immediately begin updating their credence accordingly. For a human, this can happen so fast as to be unconscious; if someone suggests to me an absurd but novel hypothesis, for example that I am currently on fire or dead, I don't consciously assign a prior of 0.5 and then begin testing the hypothesis to refine my credence. Instead, I'm immediately aware that the hypothesis contradicts a wide body of available evidence, and hence my first conscious impression of the probability of the hypothesis is that it is very unlikely--but not zero!

Let me emphasize: for the hypothesis "I am currently dead", my current credence is not exactly zero, although of course it is low (maybe 1e-15?). And again, I contend that any agent that assigns a zero credence to that (or any other) hypothesis is not reasoning soundly. Among other sources of uncertainty, in this case, even defining the word "dead" is challenging.

Now, for a novel hypothesis for which one currently lacks evidence, it is not necessary to use 0.5 in particular as the prior. In fact, any prior in the open interval between 0 and 1 will suffice, so long as one then diligently and accurately collects relevant evidence and applies some procedure like Bayesian inference to update their credence. Given sufficient evidence, any soundly rational agent should converge at the same credence, regardless of the very first assigned prior probability. The only advantage of 0.5 in particular is that it, arguably, leads to faster convergence across a broad range of hypotheses.

For the specific hypotheses of capacity for psychic mind reading or the existence of extra-terrestrial technological civilizations (ETTCs), these only seem unlikely due to our current cultural context, but they are not a priori absurd or unlikely at all. I can't read your mind, but with an EKG I can measure your heart in a way someone 1000 years ago could never have imagined. I don't know if ETTCs exist, but a priori, the idea that they do should not be any more absurd than the notion to Europeans 1000 years ago that America existed and people lived there. If someone in the middle ages truly assigned a zero prior to the possibility of EKGs or people on another continent, they would be forever unable to add those facts to their knowledge of the world, even after coming into direct physical contact with those objective realities.