Aristotle viewed causes as having four factors - material, formal, efficient, and final. The material and formal causes are intrinsic, while the efficient and final causes are extrinsic. Later philosophers like Mill defined cause as the sum of invariable antecedents. The view of causation has passed through three stages - the popular stage focused on dominant causes, the scientific stage distinguished between causes and mere coexistence, and the speculative stage viewed both causes and effects as complex. Causes can combine in mechanical or chemical ways, producing homogeneous or heterogeneous effects. Cause and effect also often interact in a relationship of mutuality.