On Peace of Mind (or "Accurate Self Assessment")

"We ought, however, first to examine our own selves, next the business which we propose to transact, next those for whose sake or in whose company we transact it.

It is above all things necessary to form a true estimate of oneself, because as a rule we think that we can do more than we are able: one man is led too far through confidence in his eloquence, another demands more from his estate than it can produce, another burdens a weakly body with some toilsome duty. Some men are too shamefaced for the conduct of public affairs, which require an unblushing front: some men's obstinate pride renders them unfit for courts: some cannot control their anger, and break into unguarded language on the slightest provocation: some cannot rein in their wit or resist making risky jokes: for all these men leisure is better than employment: a bold, haughty and impatient nature ought to avoid anything that may lead it to use a freedom of speech which will bring it to ruin. 

Next we must form an estimate of the matter which we mean to deal with, and compare our strength with the deed we are about to attempt: for the bearer ought always to be more powerful than his load: indeed, loads which are too heavy for their bearer must of necessity crush him: some affairs also are not so important in themselves as they are prolific and lead to much more business, which employments, as they involve us in new and various forms of work, ought to be refused. 

Neither should you engage in anything from which you are not free to retreat: apply yourself to something which you can finish, or at any rate can hope to finish: you had better not meddle with those operations which grow in importance, while they are being transacted, and which will not stop where you intended them to stop."  (Seneca, "On Peace of Mind")