Both psychologists and linguists challenged Krashen's model. Linguist Lydia White (1987) questioned one of his hypotheses in paper called 'Against Comprehensible Input'. Psychologist Barry McLaughlin' 1978 article was one of the first to raise the question of whether the five hypotheses could be tested by empirical research. For example, distinguishing between ' acquired' and 'learned' knowledge can lend to circular definitions (if it's acquired, it's fluent; if it's fluent, it's acquired) and to a reliance on intuition rather than observable differences in behaviour.