I think you're taking the quote the wrong way. I believe there is an implicit "work to" that isn't being stated. 'Never (work to) memorize something you can l

I think you're taking the quote the wrong way. I believe there is an implicit "work to" that isn't being stated. 'Never (work to) memorize something you can look up.' This has the meaning of, let your (short term memory) figure out based on how often you look something up if it should be remembered or not.