Lambert, Johann Heinrich (yōˈhän hĪnˈrĭkh lämˈbĕrt) [key], 1728–77, German-French philosopher and scientist, b. Alsace. He developed many basic concepts in mathematics, including that of the hyperbolic functions in trigonometry. In physics he achieved valuable results in work on the measurement of the intensity of light (the metric unit of brightness in the cgs system is named for him), degrees of heat, and humidity. In his philosophical work Neues Organon (1764) he pointed out the importance of beginning with experience and using the analytical method to investigate any theory of knowledge. His correspondence with Kant is of great philosophical significance. His other important books are Photometria (1760) and Pyrometrie (1779).