There must be some wisdom in the folk saying: Its the strong swimmers who drown.

You need a different checklist and different mental models for different companies. I can never make it easy by saying, 'Here are three things.' You have to derive it yourself to ingrain it in your head for the rest of your life.

What matters most: passion or competence that was born in? Berkshireis full of people who have a peculiar passion for their own business. I would argue passion is more important than brain power.

In my life there are not that many questions I can't properly deal with using my $40 adding machine and dog-eared compound interest table.

Over the long term, the eclipse rate of great civilizations being overtaken is 100%. So you know how it's going to end. (Laughter) I'm more optimistic about the staying power of what's good in this country. But just because you have a wonderful spouse doesn't mean you should treat her badly. You have the feeling that some of the old virtues [that made this country great] are lessening. But there's so much good and so much strength left that I would not expect this country to suddenly founder.

The ethical rule is from Samuel Johnson who believed that maintenance of easily removable ignorance by a responsible office holder was treacherous malfeasance in meeting moral obligation. The prudential rule is that underlying the old Warner & Swasey advertisement for machine tools: "The man who needs a new machine tool, and hasn't bought it, is already paying for it". The Warner & Swasey rule also applies, I believe, to thinking tools. If you don't have the right thinking tools, you, and the people you seek to help, are already suffering from your easily removable ignorance.

Obviously, consideration of costs is key, including opportunity costs. Of course capital isn't free. It's easy to figure out your cost of borrowing, but theorists went bonkers on the cost of equity capital. They say that if you're generating a 100% return on capital, then you shouldn't invest in something that generates an 80% return on capital. It's crazy.