Conventionalism asserts that there are a variety of notions of justice, but no true one. The fundamental laws of any given society are said to be grounded, not on external considerations of natural right, but human agreements which change from society to society and age to age. Justice is viewed as arbitrary and the best regime a fiction. Political society is an artificial, not natural, means to achieve man's true end--individual pleasure. Thus the crucial problem raised by conventionalism is whether political society exists by convention or nature. This dissertation examines the central claim of conventionalism, namely, whether human beings gather together into political society by convention or nature. The former argument is given to the Roman Epicurean Lucretius; the latter, the Roman Academic Cicero.