Meditations, Book 8, Section 39

Marcus Aurelius — The Emperor

Now apply this to yourself. Are you caught up in pain or pleasure? Let your senses deal with that. Have you hit some obstacle in what you're trying to do? If you set your goal without expecting setbacks, then yes, your mind has taken a hit. But if you generally expect that anything might happen, you're not really hurt or blocked. In the things that truly belong to the mind, no one can stop you. Fire can't reach it. Neither can weapons, tyrants, or cruel words. Nothing else can get inside.

Once the mind becomes whole and solid, there's no fear it will ever change.

Marcus Aurelius — The Emperor Original

Now apply all those things unto thyself. Do either pain or pleasure seize on thee? Let the senses look to that. Hast thou met with Some obstacle or other in thy purpose and intention? If thou didst propose without due reservation and exception now hath thy reasonable part received a blow indeed But if in general thou didst propose unto thyself what soever might be, thou art not thereby either hurt, nor properly hindered. For in those things that properly belong unto the mind, she cannot be hindered by any man. It is not fire, nor iron; nor the power of a tyrant nor the power of a slandering tongue; nor anything else that can penetrate into her.

If once round and solid, there is no fear that ever it will change.