Suffering from lifelong melancholy, Roget found that meticulously categorizing words provided a comforting sense of order to his obsessive-compulsive tendencies.

As a physician, Roget was inspired by biological taxonomy. He originally classified words into six broad conceptual categories, such as Matter, Intellect, and Space.

Derived from the Greek word "thesauros," the term originally described a storehouse of precious items before Roget applied it to a curated collection of words.

These words, known as synonyms, help writers avoid repetition. Unlike a dictionary that defines terms, a thesaurus offers diverse ways to express the exact same idea.