When a man overlooks even that which he does know.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“This I think every one must confess, especially if he considers the usual cause of this wrong judgment, whereof these following are some:— 69. Causes of this. (i) IGNORANCE: He that judges without informing himself to the utmost that he is capable, cannot acquit himself of judging amiss. (ii) INADVERTENCY: When a man overlooks even that which he does know. This is an affected and present ignorance, which misleads our judgments as much as the other. Judging is, as it were, balancing an account, and determining on which side the odds lie. If therefore either side be huddled up in haste, and several of the sums that should have gone into the reckoning be overlooked and left out, this precipitancy causes as wrong a judgment as if it were a perfect ignorance.” source