Galen, working in the 2nd century, developed theories that were rooted in the humoral theory of medicine, which posited that the body was governed by four humors: blood, phlegm, black bile, and yellow bile. He believed that cancer was primarily caused by an excess of black bile, which was thought to be a cold and dry humor. This theory aligned with observations of tumors being hard and unyielding, characteristics associated with the nature of black bile.
