It is unlikely that extraterrestrials have ever visited earth

This argument occured to me a few years ago, and although I've shared it with various people, this is the first time I've really subjected it to scrutiny from a person attempting to debunk it. I appreciate that my opponent has not dropped the ball.In the last round, I gave a syllogism to summarize my argument. Then I defended each premise individually. My opponent did not question the validity of my argument. Her critique concentrates solely on the truth of the premises. Nothing she said gave us any reason to think my premises were false. Rather, if her critiques are valid, that would just mean I have failed to show that my premises are true. So her critiques are undercutting defeaters rather than rebutting defeaters.[1]Since she did not question the validy of my argument, we have only to look at how she attempted to undercut the premises.1. If any extraterrestrials have ever visited earth, then they probably live within 50 light years of earth.The crux of Con's critique is that my argument depended on a lot of unwarranted assumptions. Let's look at them one at a time.assumption 1: we can guess at extraterrestrials' decision-making processesActually, we don't need to look at their decision-making process at all. We know, by the law of excluded middle, that if they came here, they either did it for a reason or they did not do it for a reason. If there was no reason, then the odds are against them coming here because there are hundreds of billions of solar systems in the galaxy. If they did it for a reason, then there had to be something that distinguished earth from everywhere else. The only thing that distinguishes earth is that there is intelligent life on earth.assumption 2: extraterrestrials would have visited Earth only while humans were aliveActually, I did not assume this; rather, I gave an argument for it. I argued that the only reason aliens would single earth out is because there is intelligent life on earth and the aliens knew about it.assumption3: extraterrestrials are made of matterThis was not an assumption. This was a stipulation of the debate. If you'll recall in the first round, I said that by "extraterrestrials," i meant "living intelligent beings who originate somewhere in the universe other than earth," and that by "living," I meant "living in the biological sense, so I'm not talking about angels, demons, ghosts, spirits, or gods." If an alien is living in the biological sense, then they are made of matter because life, in the biological sense, is composed of complex chemistry.assumptions 4 and 5: extraterrestrials live on planets and are roughly the same size scale as usAlthough it's true that I was imaginging aliens who lived on planets and were roughly the same scale as us, it's really not relevant to my argument. Pro suggests that perhaps they could've been nomads. But again, they either wandered here aimlessly, which the odds are against, or they came here on purpose because they knew there was intelligent life here.Con also suggests that perhaps they could even be several light years wide. Recall that my resolution was that it is unlikely that extraterrestrials have ever visited earth. While one might entertain the mere possibility of a biological entity being several light years wide, just because something is possible doesn't mean it's reasonable to believe. But in this case, Con's suggestion is not even possible. No biological entity could be several light years wide because the gravity would crush it, making chemistry impossible. Think about it. An organism that large would be bigger than the biggest star ever discovered. And how could the earth survive a visit from such a thing?Besides that, the size of an alien is irrelevant to my argument.2. It is unlikely that any extraterrestrials capable of traveling to earth live within 50 light of earth.Only one of the assumptions was relevant to my second premise:assumption 6: we would be able to perceive extraterrestrials, perhaps by SETIThis was not an assumption. I actually gave an argument for it. The argument is that any civilization capable of traveling to other solar systems would be more advanced than ours, and since we have already discovered radio technology, they have probably discovered it long before us. And if they have discovered it, and if they live within 50 light years of earth, then SETI would've discovered them already.Con's only objection is that we have no reason to suspect aliens would have any use for radio technology. At best, all Con can say is that it's merely possible that visiting aliens don't use radio technology, but as I said before, just because something is possible doesn't mean it's reasonable to believe. Remember that I am only arguing that it's unlikely that extraterrestrials have ever visited earth, not that it's impossible. Given the fact that we have gotten a great deal of use out of radio technology, we have a strong inductive reason to believe aliens would, too. Radio technology allows us to communicate over long distances wirelessly, as well as operate machinery over long distances. That would be very useful to any advanced civilization, and their advancement would be greatly impeded if they did not have it.ConclusionKeep in mind the resolution: It is unlikely that extraterrestrials have ever visited earth. I am not saying it is impossible, so it is not enough to defeat my argument for my opponent to give us a far-fetched scenario that is merely possible.I want to thank my opponent once against for having this debate with me which has allowed me to subject my argument to scrutiny. I've always wondered how people might object.I also want to thank the readers for coming to tonight's debate. I hope you found it interesting and that it made you think. I also ask that you vote, though I will not be so crass as to ask you to vote for me. Vote for who you think won.[1] This web page explains the distinction: http://www.iep.utm.edu...