The Enchiridion, by Epictetus: III

In everything which pleases the soul, or supplies a want, or is loved, remember to add this to the (description, notion): What is the nature of each thing, beginning from the smallest? If you love an earthen vessel, say it is an earthen vessel which you love; for when it has been broken, you will not be disturbed. If you are kissing your child or wife, say that it is a human being whom you are kissing, for when the wife or child dies, you will not be disturbed.