Answer: Along with Karl Marx, Albert Einstein, Sigmund Freud was a major influence of 20th century thought.
Even though Marx published his ideas, most notably in his book, The Capital, during the 19th century, his ideas concerning socialism and communism marked almost every political struggle in the 20th century. As for Einstein, from early on in the 20th century, his ideas concerning the relativity of time changed mathematics and physics forever. And finally, Freud changed the way the mind was understood through his psychoanalytical theory that views the mind as a tripartite complex: the Id, the Ego, and the Superego.