The Definition of Chalcedon (451 AD) is the definitive Christological formula of mainstream Christianity: ‘one and the same Christ, Son, Lord, Only-begotten, recognized in two natures, without confusion, without change, without division, without separation.’ The two natures — divine and human — are united in one person (hypostasis) and one subsistence (hypostasis again — the same word serves two functions). The four negatives define orthodoxy by exclusion: ‘without confusion, without change’ rules out Eutychianism/Monophysitism (the natures merging into one); ‘without division, without separation’ rules out Nestorianism (the person splitting into two). Chalcedon is accepted by Rome, Constantinople, and most churches that become Catholic and Orthodox. It is rejected by the Oriental Orthodox churches (Coptic, Ethiopian, Syriac, Armenian) who hold a Miaphysite position.