Gustav Fechner was a German philosopher and psychologist, born on April 19, 1801. He is best known for founding the field of psychophysics, which studies the relationship between physical stimuli and the sensations and perceptions they produce. Fechner's work laid the groundwork for modern psychology and influenced later thinkers like Wilhelm Wundt. Fechner introduced the concept of the Weber-Fechner Law, which quantifies how changes in stimulus intensity relate to changes in perception. His influential book, "Elements of Psychophysics," published in 1860, established key principles that are still relevant in psychology and sensory research today.