See yourself and others objectively. People should do what they're good at. Nobody can do everything well.

On the page 24 of "Principles", Ray wrote:

Let's imagine that your goal is to have a winning basketball team. Wouldn't it be silly to put yourself in a position that you don't play well? If you did, you wouldn't get what you want. Whatever your goals are, achieving them works the same way.

(snip)

... nobody can do everything well.

The biggest mistake most people make is to not see themselves and others objectively. If they could just get around this, they could live up to their potentials.

Note: boldface and underline indications in the first paragraph by the author of this article