As a scientist in the 19th century, Ernst Mach would make contributions to our understanding of the world that many others would build on; as a philosopher, though, his efforts at laying this groundwork were even more effective. Born in 1838, Mach entered the University of Vienna at age 17 and had his doctorate by the time he was 22. While he long identified as a physicist, he found himself increasingly drawn to elements of psychology and philosophy. Starting in 1867, he conducted a series of experiences to better understand kinesthetic sensation, or the feeling associated with movement and/or acceleration. This area was of particular importance to him because, philosophically, he believed that knowledge only ever derives from sensation.
By the early 1870s, he was doing additional research into sound waves, ultimately developing optical techniques to effectively measure sound waves, including their genesis and termination. In 1887, his most well-known contribution to the realm of science was made: his principles of supersonics incorporated something that became known as the Mach number, or the ratio of an object's velocity to the velocity of sound. Though he would retire from active scientific research in 1901, his contributions would be built upon by Albert Einstein when developing his theory of relativity. His last years were marked by writing, publishing and speaking, including an autobiography in 1910. He would live to see his 78th birthday in 1916, dying just one day afterwards.