“Until philosophers rule as kings in cities or those who are now called kings and leading men genuinely and adequately philosophize, that is, until political power and philosophy entirely coincide, there will be no end to the evils for cities”Platon · Republica · -375 · 473d
Cities escape their evils only when power and wisdom rest in the same hands.
Platon sees political evil as the result of separating power from knowledge: those who rule usually chase personal gain or glory, while those who understand truth avoid public life. The mechanism is simple — good decisions require knowing the good, not merely wanting to command. A minister who ignores education data but promises popular reforms repeats exactly the problem the phrase points to.
Why it matters Useful when judging candidates or leaders: ask whether they have real competence or merely an appetite for power, and who understands the problems but does not want the office.