Contracts are fundamental toward characterising the very nature of a firm (or enterprise). The firm is considered by some economic theories as a bundle of contracts and contracts in turn are considered also as bundles of rights and obligations (commitments). As such it can be argued that the ontological relationships between the firm and its contracts an be explained through a set of mereological (or whole-part) relationships. Specifically, the relationships between a contract and its parties and between the parties and their rights/commitments are all mereological. This view of what contracts are may appear at first surprising but a perdurantist interpretation of contracts results in such an ontology. The main contribution of this paper is a perdurantist ontology of contracts which introduces the following distinctive features: (1) a differentiation between contract specification and contract execution, (2) contract executions as objects whose spatio-temporal extents intersect those of its parties and (3) a generic model of contractual commitments and fulfilment events impacting the economics of the enterprise. The ontology proposed in this paper is applied to an example scenario to demonstrate its benefits in enterprise modelling.