It is not the cares of today, but the cares of tomorrow, that weigh a man down.

The saying contrasts present, manageable responsibilities with the imagined burdens of the future. MacDonald’s point is that anxiety is often less about what must be done now than about projecting ourselves into “tomorrow,” rehearsing possible failures, losses, or obligations before they arrive. By treating anticipated troubles as if they were already real, a person doubles the load: today’s work remains, while tomorrow’s hypothetical weight is carried prematurely. The line thus counsels a discipline of attention—meeting the day’s duties without borrowing distress from the future—and implies that much suffering is self-generated by fearful anticipation rather than by immediate circumstance.