OntoClean is a methodology for validating the ontological adequacy and logical consistency of taxonomic relationships. It is based on highly general ontological notions drawn from philosophy, like essence, identity, and unity, which are used to elicit and characterize the intended meaning of properties, classes, and relations making up an ontology. These aspects are represented by formal metaprop- erties, which impose several constraints on the taxonomic relationships between con- cepts. The analysis of these constraints helps in evaluating and validating the choices made. In this chapter we present an informal overview of the philosophical notions involved and their role in OntoClean, review some common ontological pitfalls, and walk through the example that has appeared in pieces in previous papers and has been the basis of numerous tutorials and talks.