Ontario Institute for Studies in Education, 252 Bloor Street, Toronto, Ontario, Canada M5S 1V6 People often know rapidly and reliably that they do not know something. A review of contemporary theory reveals that the issue has not received much attention; moreover, the accounts available assume that recognition that one does not know something is achieved only by failure to establish that one does know it. A reaction time experiment assessed two aspects of knowing not, by asking people whether they knew common nouns well enough to use them in sentences and whether they had visited certain cities. The findings were that affirmations of negation were often more rapid than positive reports; hence, the account of knowing not as the complement of knowing that something is the case is not necessarily correct. We suggest that knowing not may be attained as rapidly as positive knowledge on the basis of ability to carry out analytical procedures.