When thinking is relevant, it is focused on the main task at hand.

It selects what is germane, pertinent, related. It is on the alert for everything that connects to the issue. It sets aside what is immaterial, inappropriate, extraneous, beside the point. That which directly bears upon (helps solve) the problem you are trying to solve is relevant to the problem. When thinking drifts away from what is relevant, it needs to be brought back to what truly makes a difference. Undisciplined thinking is often guided by associations (this reminds me of that, that reminds me of this other thing) rather than what is logically connected (“If a and b are true, c must also be true”). Disciplined thinking intervenes when thoughts wander and concentrates the mind on the things that help it figure out what it needs to figure out.

If you find your thinking digressing, try to figure out why. Is your mind simply wandering? If so, you probably need to intervene to get it back on track. Or perhaps you realize that you need to deal with a different issue before addressing the one you were originally focused on. If so, by all means address the issue your mind has surfaced. But most importantly, know precisely, at any given moment, the issue you are addressing. And then stick to that issue until you have either reached resolution or made an active decision to revisit the issue later, or deal with the alternative issue that has emerged and stick to that issue. But do not allow your mind to wander aimlessly from idea to idea, issue to issue, without direction or discipline.