In the foreword to the first edition, Dr. Roget described his thesaurus as:
Roget's Thesaurus is composed of six primary classes. Each of these classes is itself composed of multiple divisions and then sections. This may conceptually be seen as a tree contain over a thousand branches for individual 'meaning clusters' or semantically linked words. These words are not exactly synonyms, but can be viewed as colours or connotations of a meaning or spectrum of a concept. One of the most general words is chosen to typify the spectrum as its headword, which is indexed for the whole group.
Roget's Thesaurus can be seen as a classification system as evidenced by the /Outline from the public domain 1911 /Edition?