A foundation ontology purports to describe "what exists", to a sufficient degree of rigor to establish a reasonable method of empirical validation. Acceptance of these tends to vary drastically from culture to culture: classical Greek and Roman civilization assumed for example that "earth, air, fire, and water" sufficiently described the elements, while 19th century scientists considered the periodic table to be a solid foundation ontology describing all atoms that could exist.
No foundation ontology seems to be universally accepted by all peoples. Within the physics community, the two most common foundation ontologies are the reductionist position, which is held most strongly by particle physicists, and the anti-reductionist position, which tends to be held by solid state physics. The reductionist position is that one can understand the universe by examining its most basic components and how they interact and from this understanding derive (even only in principle) an understanding of how the entire universe works. The anti-reductionist position is that collections of objects sometimes exhibit behaviors which are independent of the objects themselves. Therefore it is incorrect to think of the objects as more fundamental than the collections of objects.
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