Pius XII gave two reasons why the Church gives the highest form of worship to the Heart of Jesus. The first rests on the principle whereby the believers recognise that Jesus' Heart is hypostatically united to the "Person of the Incarnate Son of God Himself". The second reason is derived from the fact that the Heart is the natural sign and symbol of Jesus' boundless love for humanity. The encyclical recalls that for human souls the wound in Christ's side and the marks left by the nails have been "the chief sign and symbol of that love" that ever more incisively shaped their life from within.