"The rule is perfect: in all matters of opinion our adversaries are insane"

This quote by Mark Twain is a funny way of expressing the concept that when it concerns matters of viewpoint, it is impossible to come to a consensus. Everyone has their own viewpoint and no one can be proven ideal or wrong. Twain is recommending that when we disagree with someone, it is easy to consider them as being "ridiculous" or unreasonable, when in reality, they are just as entitled to their viewpoint as we are to ours. This quote is a tip that we should respect the opinions of others, even if we don't concur with them. It is necessary to keep in mind that everyone is entitled to their own viewpoint and that it is not our location to judge them for it.