“If I succeed in mentally constructing a fortress from which it is impossible to escape, this conceived fortress either will be the same as the real one - and in this case it is certain we shall never escape from here, but at least we will achieve the serenity of one who knows he is here because he could be nowhere else - or it will be a fortress from which escape is even more impossible than from here - and this, then, is a sign that here an opportunity if escape exists: we have only to identify the point where the imagined fortress does not coincide with the real one and then find it.”