Complex sentence: Dr. Orison Swett Marden (1850-1924) was an American inspirational author who advocated self-help as the foundation of success and whose ideas were based upon New Thought philosophy.
Split into simpler sentences: Dr. Orison Swett Marden (1850-1924) was an American inspirational author who encouraged individual self-development and expounded upon principles of successful living. He believed in self-reliance as the foundation of success and based many of his ideas on New Thought philosophy.