Plato's problem that Chomsky applies to speakers' knowledge of word meaning concerns the disparity between impoverished evidence given and perfect knowledge attained. This study explored whether the attained knowledge of word meaning is stable, articulated and known to adult speakers of Japanese. 118 university students judged twice the degree in which they knew the meanings of words (knowledge score) and the degree in which they could use the words in sentences (usage score), with two different intervening tasks being given between the two judgments. Some speakers engaged in a task of defining the meaning of the words and using them in sentences, while others did not. Findings showed that knowledge scores were comparable in the first judgments, while knowledge score decreased in the second judgments only for the speakers given a task of definition. No change was found in usage score regardless of the intervening tasks. Analyses of descriptions provided in the task of definition and use showed that the meanings of words were neither articulated nor known in detail to the speakers. These findings are not compatible with Plato's problem.

Key words: word meaning, Plato's problem, definition, knowledge and use