The importance of having friends who Disagree

Everyone knows the Importance of Havin Friends but what about the importance of having friends who Disagrees with the stuff you say.

Paul Graham says, in one of his essays (emphasis mine) "....Why do you need other people? Can't you just think of new ideas yourself? The empirical answer is: no. Even Einstein needed people to bounce ideas off. Ideas get developed in the process of explaining them to the right kind of person. You need that resistance, just as a carver needs the resistance of the wood.
This is one reason Y Combinator has a rule against investing in startups with only one founder. Practically every successful company has at least two. And because startup founders work under great pressure, it's critical they be friends....."

In my experience, this need for "friendly resistance" extends to far more than creating startups. Every time you have a new idea, you need people you can bounce it off. To get any real benefit out of this process you need people with a complex combination of characteristics.

They should

have firm (but not rigid) opinions on their own
have logical reasons for those beliefs and be able to articulate them clearly
are driven by ideas and not ideology
not attach their egos to their opinions.
be willing to concede a valid argument even if it forces them to possibly re-examine their beliefs
know how to listen
Given all that, it is extremely difficult to find such "friends who disagree". I have been fortunate to have quite a few people around me, who have major disagreements with me, but are still friends

Many people make the mistake of surounding themselves with people who think exactly like them and reinforce every idea or prejudice they have. This is a bad mistake and will often end up distorting the reality you see. One needs the corrective bucket of cold water in one's face once every so often. The only problem with surrounding yourself with bright people who think differently is that you may occasionally find that one of your ideas isn't as hot as you think it was or that one of your deeply held convictions is just plain wrong. This is dificult for some people because they make the rightness of their ideas a validation of their worth as persons. Blogging is a great way to expand this circle. But it can work both ways and one may end up with a "fan club" that just reinforces your prejudices.

So do a quick test right now. Make a list of your closest friends/acquintances/advisors. Then make a separate list of those who think totally differently from you, but are worth listening to anyway. Look at the intersection of these lists.

You may be surprised.