The First Edition of Dr. Roget's Thesaurus was published in the year 1852, and a second in the ensuing spring. On the issue of the third, in 1855, the volume was stereotyped. Since that time until now, the work has been reprinted in the same form and with little alteration, in rapidly succeeding editions, the printing of which has worn out the original plates.During the last years of the author's life, which closed, at a very advanced age, in the month of September, 1869, he was engaged in the task of collecting additional words and phrases, for an enlarged edition which he had long projected. This he did not live to complete, and it became my duty, as his son, to attempt to carry the design into execution.The result of the author's labours was embodied in a copy of the Thesaurus, in which the margins and spaces about the letter-press were closely covered with written words and phrases, without any very precise indication of the places in the text where additions or alterations were intended to be made. On a careful examination of these addenda, I came to the conclusion that, in order to introduce them with advantage, it would be necessary to make some slight changes; without, however, interfering at all with the framework of the book, and but little with the details of its system. In this proceeding my course has been mainly determined by the following considerations.Any attempt at a philosophical arrangement, under categories, of the words of our language, must reveal the fact that it is impossible to separate and circumscribe the several groups by absolutely distinct boundary lines. There will always be found to exist, between the words in one group and those in another, a variety of mutual affinities, corresponding with similar relations among the ideas expressed. Many words, originally employed to express simple conceptions, are found to be capable, with perhaps a very slight modification of meaning, of being applied in many varied.