Would it be sensible to say "I know Santa does not exist", and more generaly, what do we really say when we say "I know that [some fact]" ?
It is a well known principle that "absence of evidence is not evidence of absence". As such, we have no evidence Santa does exist, but also no evidence he does not.
Yet, I feel comfortable saying I know that Santa does not exist, below are my reasons and I would like to know if the reasoning is sound, and if possible references of authors who have written on the topic.
To be specific, let's consider we speak about the classic Santa, who has a base at the north pole, knows who have been naughty, receives letters and deliver presents to all the kids, during the night from 24th December to 25th, directly in their home by the way of a flying sled.
This Santa surely breaks all the laws of physics, and the probability that he can fly a sled at mach 30 at low altitude without notice is incredibly low. Nothing of the sort has ever been witnessed as far as we know. If we take as a definition of "I know X"="I hold X to be true and I have good reasons for it", then I surely hold the inexistence of Santa to be true, and i have as good reasons as can be for it.
I heard the argument that these are good reason to not believe Santa exists, but you can't say you know. However improbable, it's impossible to totally rule out the possibility that he has a crazy secret technological or magic trick that lets him deliver the presents every Christmas. There is still a probability that I am wrong, still a possibility that some day someone produces evidence for Santa and I will have to change my position. Therefore "not believe" is said to be the correct phrasing.
Yet, if i compare with other facts I know, and nobody would ever contradict me if I say I know them, I can't see the difference.
For example if I say "I know I was born the 12th of March", nobody would ever dare to say "oh no, you believe you are born the 12th, but there is still a chance you might be wrong". Yet, if we are to be rigorous about it, it is still possible, albeit improbable, that someone someday brings official papers, the testimony of the midwife, the maternity archives demonstrating that I was in fact born the 15th, and I was wrong this whole time. Administrations make mistakes, so there is a possibility that I will someday have to change my position about this fact I know.
To be honest, I would be less surprised to learn that I was mistaken about my birthday, than I would be surprised to meet Santa. As in orders of magnitude less surprised. So if I can say "I know i was born the 12th of march", why can't I say "I know Santa does not exist" ?
Edit: This question is not so much about "absence of evidence is not evidence of absence", but the value of the expression "I know that...". Why is it trivially accepted for some facts (birthday), and usually rejected for others (the existence of Santa) ? Is there some kind of epistemological difference between the two facts, that justify this, or are people just shying away from what they perceive as too bold a statement ? And if the mere criterion that I think I have good reasons is enough to justify me saying "I know Santa does not exist", how is it different from a madman who thinks he has good reasons to believe there is a pink dragon in his garage ?