Roman prayers and sacrifices were often envisioned as legal bargains between deity and worshipper. The Roman principle was expressed as "do ut des": "I give, so that you may give." Cato the Elder's treatise on agriculture contains many examples of preserved traditional prayers; in one, a farmer addresses the unknown deity of a possibly sacred grove, and sacrifices a pig in order to placate the god or goddess of the place and beseech his or her permission to cut down some trees from the grove.