In 1975 the Clarendon Press at Oxford published Peter Nidditch's edition of John Locke's An Essay concerning Human Understanding. In his Introduction Nidditch says that his edition “offers a text that is directly derived, without modernization, from the early published versions; it notes the provenance of all its adopted readings (some of which are new, correcting long-established errors); and it aims at recording all relevant differences between these versions”. As Nidditch goes on to acknowledge, the “relevant differences” were many, “requiring several thousand registrations both in the case of material variants (deletions, additions, or changes of wording) and in the case of formal variants (changes of punctuation, parentheses, italics, etc.)”. The textual history of Locke's Essay is extremely complicated. While there is no manuscript of the first edition of the book, there were four editions in Locke's lifetime, each new one containing extensive and significant revisions, as well as a posthumous edition published shortly after the author's death. There was a translation into French made with Locke's cooperation and published in 1700, and a Latin translation came out a year later. Nevertheless, Nidditch managed to record all the material variants in footnotes to the text, in a way that makes it fairly easy to track the changes that Locke made to successive editions of the book, and to locate points at which judgements had to be made as a critical text was established on the basis of the chosen copy text. Sometimes a critical edition succeeds in completely changing the way that a text is read. Peter Laslett's 1960 edition of Locke's Two Treatises of Government is a good example. Nidditch's edition of the Essay did not have that kind of very dramatic effect on Locke scholarship. Rather, it made it possible for those without direct access to all the early editions to engage in careful, historically sensitive studies of Locke's account of human understanding. The result was a slow revolution in Locke studies that continues to shed new light on even the most familiar aspects of the Lockean philosophy.