Complex sentence: Type Ontological Reductionism is the idea that every type of item is a sum type of item, and that every perceivable type of item is a sum of types of items at a lower level of complexity.
Split into simpler sentences: Type ontological reductionism is the idea that every type of item is a sum (of typically less complex) type (s) of item (s). For perceivable types of item, it says that every perceivable type of item is a sum of types of items at a lower level of complexity.