This is a great example of Peter Bevelin pulling complementary quotes together from different Warren Buffett letters to shareholders in his new book, A Few Lessons for Investors and Managers. The book also includes thoughts from Peter that add to the discussion.
“Though the mathematical calculations required to evaluate equities are not difficult, an analyst - even one who is experienced and intelligent - can easily go wrong in estimating future "coupons." At Berkshire, we attempt to deal with this problem in two ways. (1992)
First, we try to stick to businesses we believe we understand. That means they must be relatively simple and stable in character. If a business is complex or subject to constant change, we're not smart enough to predict future cash flows. Incidentally, that shortcoming doesn't bother us. What counts for most people in investing is not how much they know, but rather how realistically they define what they don't know. (1992)
If we have a strength, it is in recognizing when we are operating well within our circle of competence and when we are approaching the perimeter. (1999)
Intelligent investing is not complex, though that is far from saying that it is easy. What an investor needs is the ability to correctly evaluate selected businesses. Note that word "selected": You don't have to be an expert on every company, or even many. You only have to be able to evaluate companies within your circle of competence. The size of that circle is not very important; knowing its boundaries, however, is vital. (1996)”