Fate and necessity are unconquerable. [ Joubert ]

What though the field be lost?
All is not lost; the unconquerable will,
And study of revenge, immortal hate,
And courage never to submit or yield. [ Milton ]

Rely on principles; walk erect and free, not trusting to bulk of body, like a wrestler, for one should not be unconquerable in the sense that an ass is. Who then is unconquerable? He whom the inevitable cannot overcome. [ Epictetus ]

If the true spark of religious and civil liberty be kindled, it will burn. Human agency cannot extinguish it. Like the earth's central fire, it may be smothered for a time; the ocean may overwhelm it; mountains may press it down; but its inherent and unconquerable force will heave both the ocean and the land, and at some time or other, in some place or other, the volcano will break out and flame up to heaven. [ Daniel Webster ]