Hippocrates was an ancient Greek physician born around 460 BC who is considered the father of modern medicine. He rejected superstitions and believed that illness was caused by natural processes involving an imbalance of the four humors in the body. Hippocrates emphasized the importance of cleanliness, lighting, and a patient's mental state for healing. He made direct contributions to medicine by being the first to describe many diseases and conditions, categorize illnesses based on symptoms, and establish professional standards for physicians. The Hippocratic Oath, which doctors still take today, was based on his ethical principles of practicing medicine.