Irving Fisher (February 27, 1867 - April 29, 1947) was an American economist, statistician, inventor, and Progressive social campaigner. He was one of the earliest American neoclassical economists, though his later work on debt deflation has been embraced by the Post-Keynesian school. Joseph Schumpeter described him as "the greatest economist the United States has ever produced", an assessment later repeated by James Tobin and Milton Friedman. Fisher made important contributions to utility theory and general equilibrium.

In 1898, Fisher was diagnosed with tuberculosis, the same disease that had killed his father. He spent three years in sanatoria, finally making a full recovery. That experience sparked in him a vocation as a health campaigner. He was one of the founders of the Life Extension Institute, under whose auspices he co-authored the bestselling book How to Live: Rules for Healthful Living Based on Modern Science, published in 1915. He advocated regular exercise and the avoidance of red meat, tobacco, and alcohol. In 1924, Fisher wrote an anti-smoking article for the Reader's Digest, which argued that "tobacco lowers the whole tone of the body and decreases its vital power and resistance ... [it] acts like a narcotic poison, like opium and like alcohol, though usually in a less degree". Fisher supported the legal prohibition of alcohol and wrote three booklets defending prohibition in the United States on grounds of public health and economic productivity. He also defended eugenics, serving in the scientific advisory board of the Eugenics Record Office and as first president of the American Eugenics Society. When his daughter Margaret was diagnosed with schizophrenia, Fisher had her treated at the New Jersey State Hospital at Trenton, whose director was the psychiatrist Henry Cotton. Cotton believed in a "focal sepsis" theory, according to which mental illness resulted from infectious material in the roots of teeth, bowel recesses, and other places in the body. Cotton also claimed that surgical removal of the infected tissue could alleviate the patient's mental disorder. At Trenton, Margaret Fisher had sections of her bowel and colon removed, which eventually resulted in her death. Irving Fisher nonetheless remained convinced of the validity of Dr. Cotton's treatment.

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