Søren Kierkegaard is the influential philosopher known for his theory of existentialism. Often regarded as the father of this philosophical movement, Kierkegaard emphasized individual existence, personal choice, and the subjective experience of life. He argued that authentic living requires embracing the inherent anxiety and despair associated with human freedom. Through his writings, he explored themes of faith, choice, and individuality, challenging traditional religious and philosophical systems. His work paved the way for later existential thinkers, making significant contributions to discussions about meaning, authenticity, and the nature of the self in modern philosophical thought.