Thank you for your comment! I think your first point is true: the fact that I was born and I'm a human is much stronger evidence for the hypothesis that "only I am sentient" than for "all humans are sentient". As you mention, we have reasons to believe that other human beings are sentient, which is also why I have a much stronger prior belief in "all humans are sentient" than in "only I am sentient". Because of this, I have a stronger posterior belief in "all humans are sentient", in spite of much stronger evidence for "only I am sentient". 

It seems like approximately 109 billion humans have been born. Assuming no humans were born in the future (which is highly unlikely), this would require me to believe that "all humans are sentient" is 109 billion times more likely than "only I am sentient", prior to the evidence. I can see this prior being harder to defend as we expand to future lives and more species, and the number starts ballooning.

Good catch on the second point, though I can see that being an argument against any sort of argument made on the basis of one's existence.

P.S. I believe we've recently crossed the 8 billion people threshold. Don't ask me who's counting.

Thank you for the comment Ariel! I'm finding it pretty surreal learning about how much thinking has actually been done on the topic after some more digging. You are right that this entire post requires me to have been born for the probabilities to be the same as the number of births of different species, which is not necessarily true, since I might have also not been born at all.

I agree that any argument made by someone receiving the letter is not good evidence of the lottery being rigged, since they were always going to say that. Only winners will know that the lottery existed in the first place, and only they will tell you how it is likely to be rigged. So I agree that you hearing this argument from me should not be evidence of anything. By design, this seems like an argument that cannot convince another person. Is that true for a person arguing about their own life as well? It is my own sentience that makes me think I played the lottery (and could have lost, e.g. been a chicken), but that is simply an intuition I have. You might or might not have the same intuition.

I guess you could make the argument that there are many other lotteries I never knew I was part of to begin with. If there were a billion different lotteries played at birth, on average I would be expected to win a lottery, and I guess I simply ended up winning the sentience one. This would invalidate my sentience argument because of the filter.