American physician, researcher, author, and teacher best known for his essays, which contain lucid meditations and reflections on a wide range of topics in biology.

Lewis attended Princeton University, Princeton, N.J., and Harvard Medical School (M.D., 1937). He served in the U.S. Navy Medical Corps and taught at Johns Hopkins and Tulane universities and at the University of Minnesota Medical School. In 1954 he moved to New York University School of Medicine, which he left as dean in 1969 to teach in the pathology department at Yale University. From 1973 to 1983 he was president of the Memorial Sloan-Kettering Cancer Center.

Thomas' first book, The Lives of a Cell: Notes of a Biology Watcher (1974), was a collection of 29 essays originally written for the New England Journal of Medicine. His later essays were collected in The Medusa and the Snail (1979), The Youngest Science (1983), Late Night Thoughts on Listening to Mahler's Ninth Symphony (1983), and The Fragile Species (1992).