Meditations, Book II, Section 9

Meditations, Book II, Section 9:

Consider how quickly all things are dissolved and resolved: the bodies and substances themselves, into the matter and substance of the world: and their memories into the general age and time of the world. Consider the nature of all worldly sensible things; of those especially, which either ensnare by pleasure, or for their irksomeness are dreadful, or for their outward lustre and show are in great esteem and request, how vile and contemptible, how base and corruptible, how destitute of all true life and being they are.

I regard this one as especially important for maintaining perspective. It's hard to remember; harder still to translate into action based on that perspective.

There is the tendency to measure oneself against an ideal. I think this is the wrong view. Much better it is to measure oneself either according to (1) how you used to be; (2) how you might have acted overall--i.e., across all situations across time--were you not attempting to put these ideas into practice.