From The Effective Executive:
If there is any one “secret” of effectiveness, it is concentration. Effective executives do first things first and they do one thing at a time.
The need to concentrate is grounded both in the nature of the executive job and in the nature of man. …
The more an executive focuses on upward contribution, the more will he require fairly big continuous chunks of time. The more he switches from being busy to achieving results, the more will he shift to sustained efforts — efforts which require a fairly big quantum of time to bear fruit. Yet to get even that half-day or those two weeks of really productive time requires self-discipline and an iron determination to say “No.” …
But concentration is dictated by the fact that most of us find it hard enough to do well even one thing at a time, let alone two. Mankind is indeed capable of doing an amazingly wide diversity of things; humanity is a “multipurpose tool.” But the way to apply productively mankind’s great range is to bring to bear a large number of individual capabilities on one task. …
Concentration is necessary precisely because the executive faces so many tasks clamoring to be done. For doing one thing at a time means doing it fast. The more one can concentrate time, effort, and resources, the greater the number and diversity of tasks one can actually perform.