This was the first principle where moisture was the source of all things.  Thales also presented another philosophical school of thought where the cosmos was rational as it had a harmonious arrangement.  

Aristotle brought about the “first philosophy”.  It was his study of “being qua being”.  He will begin his studies by questioning the theories of others.  The first philosophy deals with the causes and principles. 

Even before this, Plato had worked on Platonism which deals with the theory of forms.  It is about the perfect models from which are derived the objects of world which are imperfect. 
                                                                                                     

Christian Wolff popularized ontology.  It is the study of being and the very nature of existence. It had lost its credibility in the following years but was reinstated again by latter day philosophers.

Another school of thought is materialism. This deals with the theory that the material and the immaterial are interconnected. In fact, the immaterial is a modification of the material.  It might even go so far as to say that it is actually material which has changed properties to come to its present state.

All these schools of thought have helped to either eliminate existing subjects of metaphysics or have brought in new topics.  Like its science metaphysics is dependent on the ever changing and constantly evolving theories. 

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