[evincarofautumn]

Feynman is wrong. Understanding something (technical skill) doesn't automatically make you good at explaining it (communication skill). Being poor at explaining something does not necessarily betray incomplete understanding.

[mechnik]

Feynman is not wrong. Technical skill does not equal understanding something. Consider an idiot savant who is technically proficient at playing piano but can't string two words together. These definitions of 'understand' may help: 1. Perceive the intended meaning of (words, a language, or speaker): "he could usually make himself understood"; "she understood what he was saying". 2. Perceive the significance, explanation, or cause of (something): "she didn't really understand the situation". http://www.google.com/search?aq=f&q=define%3Aunderstand