Complex sentence: He argues against fear of such deities by demonstrating through observations and arguments that the operations of the world can be accounted for in terms of natural phenomena--the regular, but purposeless motions and interactions of tiny atoms in empty space.
Split into simpler sentences: He argues against fear of such deities by demonstrating, through observations and arguments, that the operations of the world can be accounted for in terms of natural phenomena. These phenomena are the regular, but purposeless motions and interactions of tiny atoms in empty space.