No man ever sank under the burden of the day. It is when tomorrow's burden is added to the burden of today that the weight is more than a man can bear.

MacDonald contrasts the manageable weight of present duties with the crushing effect of anticipatory anxiety. The “burden of the day” suggests that most people can endure what is immediately required of them; what breaks resolve is mentally importing imagined future troubles—“tomorrow’s burden”—into today. The saying echoes a Christian ethic of living faithfully in the present (often associated with the biblical admonition not to worry about tomorrow), but it also reads as a psychologically acute observation about rumination and catastrophizing. Its significance lies in reframing endurance as a matter of temporal focus: suffering increases when the mind multiplies burdens by carrying them before their time.