Discourses, "Against the Quarrelsome and Ferocious" (§3)

“In this way also those who occupy a strong city mock the besiegers (and say): What trouble these men are now taking for nothing; our wall is secure, we have food for a very long time, and all other resources. These are the things which make a city strong and impregnable; but nothing else than his opinions makes a man’s soul impregnable.”