Hume, David - The Spherical Wave Structure of Matter in Space explains Hume's Problem of 'Necessary Connexion' , Causation and Skeptic ism. 'Experience only teaches us, how one event constantly follows another; without instructing us in the secret connexion, which binds them together, and renders them inseparable... What that medium is, I must confess, passes my comprehension; and it is incumbent on those to produce it, who assert that it really exists ... I cannot imagine any such reasoning. But I keep my mind still open to instruction, if any one will vouchsafe to bestow it upon me.'
With the rise and dominance of Christianity and the decline of secular philosophy in the West during the Middle Ages , Metaphysical Naturalism became heretical and eventually illegal . It was only when the political advances of the Age of Enlightenment made genuine free speech possible again that a few intellectuals (like Baron d'Holbach in the 18th Century) publicly renewed the case for Metaphysical Naturalism, under the label of Materialism . Later, with scientific advances in quantum physics, this developed into the more far-reaching doctrine of Physicalism .