Strong atheism is distinguished from the weaker atheism that only lacks belief, and from agnosticism, which holds the question unresolved or unresolvable. It rests on a naturalist picture of reality, that the universe is a natural system with nothing beyond it, and on the conviction that belief should follow the evidence, where its proponents find the evidence for gods wanting. As a definite affirmation about what does not exist, it marks the most assertive end of the range of non-belief, and this entry presents it as the position its adherents hold.
The philosopher Antony Flew, in his influential 1972 essay "The Presumption of Atheism," helped formalise the modern distinction between weak (negative) and strong (positive) atheism, though Flew's own argument favoured the weaker position, that unbelief should be the rational default absent positive evidence for God. Flew himself became one of the more notable reversals in modern debate when, in 2004, after decades as one of the twentieth century's most prominent philosophical atheists, he announced he had come to accept the existence of a deistic god on the basis of arguments from the complexity of biological information and the fine-tuning of physical constants, a change some fellow atheists attributed to arguments made on his behalf by others, a dispute that remained unresolved at the time of his death in 2010.