Disbelievers call themselves "freethinkers" or "secularists". They included John Stuart Mill, Thomas Carlyle, George Eliot and Matthew Arnold. They were not necessarily hostile to Christianity, as Huxley repeatedly emphasized. The literary figures were caught in something of a trap – their business was writing and their theology said there was nothing for certain to write. They instead concentrated on the argument that it was not necessary to believe in God to behave in moral fashion. The scientists, on the other hand, paid less attention to theology and more attention to the exciting issues raised by Charles Darwin in terms of evolution. The proof of God's existence that said he had to exist to have a marvelously complex world was no longer satisfactory when biology demonstrated that complexity could arise through evolution.