By Warren Buffett
What matters for most people is not how much they know, but how realistically they define what they don't know.
— Warren Buffett
This relates to Buffett’s “circle of competence” concept, first outlined in his 1996 Letter to Shareholders:
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.
Warren’s long-time business partner has a very similar line: