Tertullian made an important contribution to the debate in his Treatise against Praxeas. He described God as "one substance consisting in three persons." He dealt exhaustively with the unity in the Trinity and did not proceed to the further problem that arose, namely, that of the equality of rank between the divine personae. Furthermore, he was not a philosopher and it is not easy to determine the precise meaning of the Latin words substantia and persona which he used. Nevertheless his teaching represented an achievement for future orthodoxy.