True courage is a result of reasoning. A brave mind is always impregnable.
— Jeremy Collier

Most people hew the battlements of life from compromise, erecting their impregnable keeps from judicious submissions, fabricating their philosophical drawbacks from emotional retractions and scalding marauders in the boiling oil of sour grapes.
Unknown impregnable quote

Ah, I fancy it is just the same with most of what you call your emancipation. You have read yourself into a number of new ideas and opinions. You have got a sort of smattering of recent discoveries in various fields -- discoveries that seem to overthrow certain principles which have hitherto been held impregnable and unassailable. But all this has only been a matter of intellect, Miss West -- superficial acquisition. It has not passed into your blood.
— Henrik Ibsen

You think, eventually, that nothing can disturb you and that your nerves are impregnable. Yet, looking down at that familiar face, I realized that death is something to which we never become calloused.
— Eliot Ness

Temptation, if it is not to conquer, must not fall like a bomb against another bomb of instantaneous moral explosions, but against the strong walls of an impregnable fortress strongly built up, stone by stone, beginning at that distant day when the foundations were first laid.
— Maria Montessori