It means that though the Witch knew the Deep Magic, there is a magic deeper still that she did not know. Her knowledge goes back only to the dawn of Time. But if she could have looked a little further back, into the stillness and darkness before Time began, she would have read there a different incantation. She would have known that when a willing victim who had committed no treachery was killed in a traitor's stead, the Table would crack and death itself would work backwards.
C. S. LewisThe promise, made when I am in love and because I am in love, to be true to the beloved as long as I live, commits me to being true even if I cease to be in love. A promise must be about things that I can do, about actions: no one can promise to go on feeling in a certain way. He might as well promise to never have a headache or always to feel hungry.
C. S. LewisI do not think that all who choose wrong roads perish; but their rescue consists in being put back on the right road. A sum can be put right: but only by going back til you find the error and working it afresh from that point, never by simply going on. Evil can be undone, but it cannot 'develop' into good. Time does not heal it. The spell must be unwound, bit by bit, 'with backward mutters of dissevering power' --or else not.
C. S. Lewis