As a rule of thumb, commonsense is the knowledge we all have but it not necessarily recorded in reference books explicitly. For example, it is somewhat difficult to find an encyclopedia or dictionary that says that when a bowl is inverted, its contents run out. In addition to the broadly applicable information that is found in reference books, Cyc has many hand entered facts required to understand the assumptions that underlie human discourse. Commonsense representation and reasoning in the Cyc Knowledge Base has the goal of avoiding the brittleness observed when scaling up typical expert systems to include more knowledge. Cyc is engineered to eventually have a suitable representation for the full range of human expression, so that expert knowledge bases can be created by extending from the Cyc upper and middle ontology. Our methodology assumes that representing new knowledge is much easier when a large body of general purpose knowledge is already present. Cyc uses inheritance hierarchies within its major object types (e.g. terms, relationships and contexts) to concisely represent knowledge.