At Avot 2:5, Hillel teaches that a person should not trust him- or herself until the day of one's death.
The point of Hillel’s teaching here is subtle. We do trust ourselves to do the right thing, even in the face of temptation, external pressure or forgetfulness. We do so on a daily basis and on the assumption that there is no-one whom we can trust better than ourselves – but this self-belief is likely to be misplaced since the experience we share with most of humanity is that we are deluding ourselves.
Consider the following. Has it ever happened to you that, leaving home to go to synagogue, visit a friend or do some shopping, you have picked up a letter that you intended to post. Returning home later, you find the unposted letter in your pocket. And have you never been certain that you turned the light off, taken the eggs off the cooker or collected your scarf from the coat-rack, only to discover that this was not in fact the case?
Episodes of this nature happen to more or less everybody if they are honest enough to admit it – but people do not consider themselves any the less trustworthy for it: they simply rationalize their error (“I was momentarily distracted;” “I was in a terrible hurry;” “It was meant to happen”) and carry on trusting themselves.
In these situations, the only thing that has gone wrong is that we have inadvertently slipped up. We didn’t want to, but we did. Now consider what might be the case when we would actually prefer to slip up and deviate from our usual good conduct. Every individual has a yetzer hara (“Evil Inclination”), a little voice inside even the best of us that occasionally tries to persuade or cajole us into error. That voice might be telling us to do something a bit mischievous and capricious, like playing an embarrassing practical joke on a friend, or it might be encouraging us to commit an act that is quite dishonest, like finding a valuable item that belongs to someone we know but deciding not to give it back. Can we really trust ourselves to do the right thing when that little voice inside us speaks so appealingly, when we shouldn’t really be trusting ourselves even when it’s silent?
Hillel offers no quick trick or easy solution. He only asks us to be aware of our own fallibility so that we can try to guard against it.