Knowledge defines ones retained history, information, or general intellect even.

Knowledge affects how many Spells you can know at a time. With high Knowledge, not only could you re-count many memories, but make informed decisions for what lies ahead. It is a very important skill for most who intend to use Spells, as it is a study that requires much work and research, as well as practice. With low Knowledge, not only would you have short term memory and little experience to recount, but generally lower intelligence as a whole. Maybe even having difficulty reading? Or difficulty speaking? The lower it gets, the worse it'll be for you in the long run.