“If thou art pained by any external thing, it is not the thing itself that disturbs thee, but thine own perception of it, and this thou hast the power to revoke at any time. But if anything in thine own disposition gives thee pain, who hinders thee from correcting thine opinion? And even if thou art pained because thou art not doing some particular thing which seems to thee to be right, why dost thou not rather act than complain?”