I argue that, for any
[falsifiable](https://en.wikipedia.org/wiki/Falsifiability) hypothesis,
a
[prior probability](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Electrocardiography)
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.