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Thomas Sydenham published his treatise on hysteria (Epistolary Dissertation to Dr. Cole) in which he recognized the widespread nature of psychological illness and independently confirmed Carolus Piso’s observation that males were as susceptible to hysterical symptoms as females. He attributed the symptoms to an imbalance of the “animal spirits” led to disorders of the most vulnerable organs; these organs were susceptible to the accumulation of putrid humors that rendered the organs unable to purify blood. Sydenham included patients with symptoms of hypochondria (males) and depression in surveying the protean symptomatology of hysteria, and his treatise was “the most important seventeenth-century treatise on psychological medicine” (Dewhurst, 75).