Ludwig Wittgenstein was an influential Austrian-British philosopher known for his work in logic, philosophy of mathematics, philosophy of mind, and philosophy of language. He is considered one of the greatest philosophers of the 20th century. There were two stages to his thought - his early work focused on logic and language in his Tractatus Logico-Philosophicus, while his later work in Philosophical Investigations criticized traditional philosophy and focused on ordinary language. Wittgenstein had a significant influence on analytic philosophy and various fields such as logic, ethics, and psychology.