You can train your mental strength just like you train your body. If your body looks fit or ripped, it looks strong, and you can flex your muscles. So, physically, you have a certain strength. Mentally, it's the same thing. You can train your psychological strength.
The admiral, or commander in chief of a squadron, being frequently invested with a great charge, on which the fate of a kingdom may depend, ought certainly to be possessed of abilities equal to so important a station and so extensive a command.