But these spirited natures are apt to bite and devour one another; the union of gentleness to friends and fierceness against enemies appears to be an impossibility, and the guardian of a State requires both qualities.

Who then can be a guardian?
The image of the dog suggests an answer.

For dogs are gentle to friends and fierce to strangers.

Your dog is a philosopher who judges by the rule of knowing or not knowing; and philosophy, whether in man or beast, is the parent of gentleness.

The human watchdogs must be philosophers or lovers of learning which will make them gentle.