Celsus was a prominent Greek philosopher and physician who lived during the 2nd century AD. He is best known for his work, "On Medicine," which provided a comprehensive overview of medical practices and theories of his time. Celsus emphasized the importance of observation and experience in medicine, laying the groundwork for future medical studies. In addition to his medical contributions, Celsus also wrote about various subjects, including philosophy and rhetoric. His writings often contrasted with those of other thinkers, such as Galen, who followed him. Although many of Celsus's works have been lost, his influence on the development of medicine and philosophy remains significant.