Franz Mesmer was an 18th-century German physician known for developing the theory of animal magnetism, which he believed could influence health through magnetic forces. He proposed that an invisible fluid in the body could be manipulated to cure ailments, leading to the practice of mesmerism, a precursor to modern hypnosis. Mesmer's ideas gained popularity in Europe, attracting both followers and critics. His methods involved using magnets and later his hands to induce a trance-like state in patients. Although his theories were eventually discredited, Mesmer's work laid the groundwork for the study of hypnosis and the exploration of the mind's influence on health.