It is unlikely that any extraterrestrials have ever visited earth.

While Pro judges I have won the debate, I address his points below, and invite voters to judge for themselves. The Argument Itself > it's quite a bit more complicated than simply multiplying the probability of each premise to get the probability of the conclusion. However, he is right that I need better than 51% probability for each premise, and I suspect 71% is close enough. I concede the math I presented was unintentionally faulty and apologize. As I've pledged not to introduce new arguments this round, I won't produce the replacement formula here, but will note that we both agree that to support Pro's conclusion, you must have a high confidence in each of the premises, in the ballpark of 70%.The First Premise: If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth. > It is true that alien races could use remote probes and such to locate life-permitting planets.. but.. I don't think this solves any problem for Con as far as time and distance is concerned. I agree the sensors were superfluous to my argument, since you stated we could assume they could detect our radio waves if in range. The sensors would've helped to amplify and direct our signals towards alien starships. > they'd have to send that information back to the home planet Rather, only to the nearest alien starship! I find the notion compelling that aliens who toiled to design and build interstellar craft would send at least some of those ships to other solar systems. A fraction of those starships would be closer to us and thus able to detect and visit us even if their home planet remained outside that 50-light-year window. > While it's true that the life-density of the galaxy is unknown, all we need for my argument to work is for the intelligent-enough-to for-interstellar-travel-life-density of the galaxy within 50 light years or so to be low or non-existent. I raised the issue of life-density because, as it increases, so too does the likelihood that Earth had a visitor whose home is more than fifty light years away. The more aliens out there, the more time they had, and the better their ability to identify stars and planets capable of detecting life.. the more likely it is that faraway aliens have visited Earth. I do agree, "Visitors are more likely to have a home 0-50 light years away than 50-100 light years away." However, I contend you didn't show "Any visitors would very likely have a home within 50 light years." I extend all other arguments.The Second Premise: It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth. > Con suggests the possibility that advanced aliens might've stopped using radio communication. If so, then it's hard to imagine how they would communicate with their probes. I did propose an alternative--that they are using other portions of the EM spectrum. Non-radio wavelengths include microwave, infrared, visible light, ultraviolet light, x-rays, and gamma rays. Radio is such a small fraction of the total EM spectrum, that without further evidence, I'd say radio is a possibility and otherwise is a probability.> SETI (depending on how thorough SETI's searches are, and Con doesn't seem to think they're very thorough) According to the quote I posted before, SETI doesn't seem to think SETI is good at finding alien races that don't want to be found! This refutes your argument that "If there were life nearby, we would know about it due to SETI." I can't say I'm very confident in either the existance or non-existance of nearby life. I extend all other arguments.ConclusionI don't believe Pro has made his case, that each of his premises are very likely to be true. Thus, his burden of proof is not met, and so his conclusion doesn't logically follow. Still, Pro's argument was interesting and well-written, and this was a fun debate that raised more questions than it answered. I thank him for sharing and discussing it!