On our Earth, we’ve perforce learned all the knavery there is to know. - Poul Anderson

His will grew tight again. This was no age for weaklings. You had to do whatever seemed best, without letting gods or men or the lower devils deter you.

“I wonder if there are any gods at all—if they aren’t just another story.”
Tom and Owl shrank from him. But no lightning struck.

There are three Powers in the world which not gods nor demons nor men can stay, against which no magic shall prevail and no might shall stand, and they are the White Christ, Time, and Love.
From the first you may await only thwarting of your desire, and you must be careful that He and His in no way enter the struggle. This you can do by remembering that Heaven leaves lesser beings their free will, and thus does not force them into its own ways; even the miracles have done no more than leave open a possibility to men.
The second, which has more names than I myself—Fate, Destiny, Law, Wyrd, the Norns, Necessity, Brahm, and others beyond counting—is not to be appealed to, for it does not hear. Nor can you hope to understand how it exists together with the freedom whereof I spoke, any more than you can understand how there are both old gods and new. But for the wreaking of the greatest spells, you must ponder on this until you know in your inmost being that truth is a thing which bears as many shapes as there are minds which strive to see it.
And the third of the Powers is a mortal thing, therefore it can harm as well as help, and this is the one you must use.