People function through their use of two kinds of knowledge: knowledge of and knowledge how.

…Knowledge how [is] what psychologists call procedural knowledge.

…Procedural knowledge is difficult or impossible to write down and difficult to teach. It is best taught by demonstration and best learned through practice. Even the best teachers cannot usually describe what they are doing. Procedural knowledge is largely subconscious.

From The Design Of Everyday Things via 37Signals. 

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