In his "Essay" John Locke identifies two grounds of probability for a historical or natural event: the conformity of the thing with our previous experience and the number or reliability of the witnesses. Locke was careful not to transform the judgment of conformity to the ordinary course of nature into a subjective assessment of the reliability of the reported fact. But the balance between the two foundations of probability was precarious, and was questioned both by apologists and by unbelievers, who interpreted the notion of verisimilitude and its weight in the evaluation of factual matters differently.