The main thing you can be sure of your capacity to change is yourself, or so the statement says. While we may have some impact with others, at last, they have the last say in what they did. Correspondingly, we can manufacture and make and do, however at last, we have even less authority over things.
The greater the thing, the less control we really have. We have to concede that we control practically nothing. Indeed, even the Captains just settle on the critical choices. Those choices have their effect, however there are heaps of others settling on choices the distance down the line. They can’t be sure that their choice enhanced things.