What is ontology? There is a theoretical disagreement in the proper status of ontology. One group regards ontology as a tool for characterizing human knowledge; the other considers ontology to be a systematic method for representing reality. This article addresses basic questions surrounding ontology engineering and human concepts from a vantage point of cognitive science. Specifically, it is argued that upper-level ontology -- basic divisions of being -- is reflected in human cognition and affects various aspects of our everyday judgments. The authors think that this is because human cognition is organized in the way that it can help approximate the invariant structure of the environment. Possible research directions that help integrate ontology engineering, cognitive science and philosophy are also discussed.