Often in literature, the prevailing opinion is backed up by material published in journals or highly cited books. And generally to get published in a journal, you need some compelling evidence. So, if I want to venture that T.S. Eliot wrote The Waste Land as a response to his own struggles with mental illness, I can back that up with citations from journals, or published books. If I want to go even further (and get an even better mark), I can go into the letters where he first hand talks about his struggles.

Just because I can back up my opinions with published material, it still doesn't mean that my opinions are correct.

To reverse your metaphor, let's say that you think pigs can fly and I think they can't. You say that pigs can fly because you saw it happen. I say that pigs cannot fly because of these respected books on physics and pig anatomy. You might actually be correct, but I should still win an efficiently run debate.