You can think about a past event and feel ashamed, even though no one knows.
Your conscience is makes you feel so guilty and remorseful that you wan to undo something if you could, even if no one is asking you to do so. Sometimes this repentance is prompted by knowing that God knows about it and other times it is brought about just by the fear of others learning about your misdeed.
The proverb has been used throughout history to remind people of the importance of self-reflection and to emphasize the strength of the conscience.
It is thought to have originated in the 4th century BC in Ancient Greece. It is attributed to the philosopher Socrates, who is believed to have said: “The unexamined life is not worth living, but a guilty conscience needs no accuser.”