You cannot eat quinces uncooked, they are very tough and the pips are highly poisonous when raw! They contain amygdalin which is processed to hydrocyanic acid in your guts. Heat immediately destroys the amygdalin so you definitely need to cook your quinces, especially when also using the core – which you do for example when making jam. The pips contain pectin and are responsible for the thickening of the jam. You can use just the flesh in meat dishes, like duck or beef, but I find it more satisfying to use the jam or quince paste instead as the intensely flowery and naturally astringent taste is so much more amazing with sweetness. By the way: in Germany, people use the slime of the cooked pips to treat a sore throat but I have never done this as I swear by a homemade onion & honey remedy instead – which, honestly, is a lot tastier than it sounds!