I had a little bit of discussion with one of my new housemates the other night, and though he jumped through many things, one part was directly related to bad reasoning and bad defence of arguments.
First, he made the claim that we are decedents from an alien race that came to earth right before ancient Egypt was formed, and they mixed their DNA with apes to make us, human beings. When I asked what is his reason for this claim, he said that one particular theory about how Egyptians made the pyramid has been falsified, and since no scientist can explain how the pyramid were made, this is evidence for the alien race.
Of course now you think that I should have left it at that, since these claims are beyond repair messed up. But, well, not me. I at least try my best, so I told him that I do not know much about pyramids, but what he is doing is appealing to ignorance: He is backing up his claims not by providing evidence for them, but by saying that since other people are wrong, his explanation must be right.
At this point he did something that is the subject of this post, he asked: “Do you doubt that aliens might exist?”
The answer to that question as physicists have suggested is most likely no, aliens are highly likely to actually exist, but that doesn’t mean that they have ever visited us, or even to have survived long enough to realized that we exist.*
This is when I felt the need to explained something essential about reasoning: Conclusions (or different parts of them) may or may not be obvious, but no one can defend a fallacious argument based on partially obvious conclusions.
Take the following argument for example:
All insects have eight legs.
Spiders are insects.
==> Spiders have eight legs.
The conclusion is obviously true, but the argument is utterly false. If someone present such argument, and then when told that the argument is false defends it by saying “If you do not believe me, let’s go and see a spider for ourselves!”, they are abandoning the basics of reasoning.
* The following video is a good speech from Richard Dawkins in which he talks about this issue as well as science and fallacious arguments. (This particular issue comes up around 35:30)