The thought expressed in the quote above is here being put to use in a slightly different context:

"I think Metrodorus spoke excellently in that letter he addressed to his sister on the loss of her very promising son: “Every good thing for mortal beings is likewise mortal.” He was referring to the goods that people flock to; for the true good, wisdom and virtue, does not perish but is secure and everlasting. This is the one immortal thing that can accrue to mortals. Yet people are so flawed and so heedless of the destination to which the passage of each day is pushing them that they are caught by surprise when they lose something, even though on a single day they will lose everything. Those things you are considered to own are in your home, but they are not yours. Nothing is secure for the insecure, nothing is lasting and invincible for a person who is fragile. It is not only our property that has to go: it is ourselves too; and this very fact, if only we understand it, is consoling. Be calm when something goes: you must go as well."

• Seneca, Letters 98.9