Gray, considered by Darwin to be his friend and "best advocate", also attempted to convince Darwin in his letters that design was inherent in all forms of life, and to return to his faith. Gray saw nature as filled with "unmistakable and irresistible indications of design" and argued that "God himself is the very last, irreducible causal factor and, hence, the source of all evolutionary change." Darwin agreed that his theories were "not at all necessarily atheistical" but was unable to share Gray's belief. "I feel most deeply that the whole subject is too profound for the human intellect. A dog might as well speculate on the mind of Newton," he wrote. Gray was a Christian, but he was a staunch supporter of Darwin in America. He collected together a number of his own writings to produce an influential book, "Darwiniana" (1876); these essays argued for a conciliation between Darwinian evolution and the tenets of theism, at a time when many on both sides perceived the two as mutually exclusive. Gray denied that investigation of physical causes stood opposed to the theological view and the study of the harmonies between mind and Nature, and thought it "most presumable that an intellectual conception realized in Nature would be realized through natural agencies". The result of all this is that Gray distanced himself from Social Darwinism. Gray is a critical link in the history of American intellectualism, and his writings that explain how religion and science were not necessarily mutually exclusive have been considered his supreme accomplishment; thereby providing a way for believers in Creationism to consider Darwin's ideas.