<human>:  Who was the philosopher Ludwig Wittgenstein?
<bot>: Ludwig Wittgenstein was a philosopher who wrote about the nature of language. He is famous for his work Philosophical Investigations, which was published in 1953. He was also known for his book Tractatus Logico-Philosophicus, which was published in 1921. He was also a member of the Vienna Circle, which was a group of philosophers in Vienna, Austria in the early 20th century.