The problem with relying on direct practice exclusively is that knowledge that isn’t in your head can’t be used to help you solve problems. For instance, a programmer may realize a need to use a certain function to solve a problem but forgets how to write it out. Needing to look up the syntax might slow her down, but she will still be able to solve the problem. However, if you don’t have enough knowledge stored to recognize when you can use a function to solve your problem, no looking up can help you.
