The last centuries of philosophy have seen vigorous questions regarding the arguments for God's existence raised by such philosophers as Immanuel Kant, David Hume and Antony Flew, although Kant held that the argument from morality was valid. The theist response has been either to contend, as does Alvin Plantinga, that faith is "properly basic", or to take, as does Richard Swinburne, the evidentialist position. Some theists agree that none of the arguments for God's existence are compelling, but argue that faith is not a product of reason, but requires risk. There would be no risk, they say, if the arguments for God's existence were as solid as the laws of logic, a position summed up by Pascal as "the heart has reasons of which reason does not know." A recent theory using concepts from physics and neurophysiology proposes that God can be conceptualized within the theory of integrative level.