The strongest and healthiest man may be stricken down in a moment, by accident or disease.
If we take human life in the mass, we cannot fail to recognize the uncertainty of life as much as we do the certainty of death.
There is a striking passage in Addison's "Vision of Mirza," in which life is pictured as a passage over a bridge of about a hundred arches.

A black cloud hangs over each end of the bridge.

At the entrance to it there are hidden pitfalls very thickly set, through which throngs disappear, so soon as they have placed their feet upon the bridge.

They grow thinner towards the centre; they gradually disappear; until at length only a few persons reach the further side, and these also having dropped through the pitfalls, the bridge at its further extremity becomes entirely clear.

The description of Addison corresponds with the results of the observations made as to the duration of human life.
Thus, of a hundred thousand persons born in this country, it has been ascertained that a fourth of them die before they have reached their fifth year; and one-half before they have reached their fiftieth year.
One thousand one hundred will reach their ninetieth year.