The probability of magic is still vanishingly vanishingly low, but given how useful magic would be it might still be worth Harry’s time to test for it.
It’s also worth noting that the letter does not only provide several orders of magnitude jump in the prior for whether magic exists or not, but also provides a method of testing its existence that is much lower-cost than before (beforehand, Harry would have had to do some fairly strenuous things to break The Masquerade; with the letter in hand, he merely needs to send a reply.)
The probability of magic is still vanishingly vanishingly low, but given how useful magic would be it might still be worth Harry’s time to test for it.
I hereby dub this class of argument Pascal’s Muggle
It’s also worth noting that the letter does not only provide several orders of magnitude jump in the prior for whether magic exists or not, but also provides a method of testing its existence that is much lower-cost than before (beforehand, Harry would have had to do some fairly strenuous things to break The Masquerade; with the letter in hand, he merely needs to send a reply.)