Plato, Protagoras

[344d] not one who is lying down already. So it is a man apt to resist that an irresistible mischance would overthrow, and not one who could never resist anything. A great storm breaking over a steersman will render him helpless, and a severe season will leave a farmer helpless, and a doctor will be in the same case. For the good has the capacity of becoming bad, as we have witness in another poet¹ who said—“Nay more, the virtuous man is at one time bad, at another good.
”unknownwhereas the bad man has no capacity for becoming, but must ever be, what he is;