John Calvin Commentary Psalms 90:12

"So teach us to number our days, That we may get us a heart of wisdom." — Psalms 90:12 (ASV)

Teach us so to number our days. Some translate this as the number of our days, which gives the same meaning. As Moses perceived that what he had taught until now is not comprehended by human understanding until God enlightens people by His Spirit, he now turns to prayer.

Indeed, it seems absurd at first sight to pray that we may know the number of our years. What? Since even the strongest people scarcely reach the age of eighty years, is there any difficulty in calculating so small a sum? Children learn numbers as soon as they begin to talk, and we do not need an arithmetic teacher to count to one hundred on our fingers.

All the more foul and shameful, then, is our stupidity in never comprehending the shortness of our lives. Even someone who is most skilled in arithmetic, and who can precisely and accurately understand and work with millions upon millions, is nevertheless unable to count eighty years in his own life.

Surely, it is an extraordinary thing that people can measure all distances outside of themselves—that they know the distance in feet from the moon to the center of the earth, the space between the different planets, and, in short, that they can measure all the dimensions of both heaven and earth—while yet they cannot count seventy years in their own lives.

It is therefore evident that Moses had good reason to implore God for the ability to do what requires a wisdom very rare among humanity. The last clause of the verse is also worthy of special notice. By it, he teaches us that we truly apply our hearts to wisdom when we comprehend the shortness of human life.

What can be a greater proof of madness than to wander aimlessly without setting any purpose for oneself? True believers alone, who know the difference between this transitory state and a blessed eternity for which they were created, understand what the aim of their life should be. Therefore, no one can order their life with a settled mind except the person who, knowing its end—that is, death itself—is led to consider the great purpose of human existence in this world, so that they may aspire to the prize of the heavenly calling.