Complex sentence: In the "Categories", he attempts to discern all the possible things to which a term can refer; this idea underpins his philosophical work "Metaphysics", which itself had a profound influence on Western thought.
Split into simpler sentences: In the "Categories", he attempted to discern all the possible things that a term can refer to. This idea underpins his philosophical work, the "Metaphysics", which also had a profound influence on Western thought.