This work by Saint Photius the Great presents a rigorous and uncompromising defence of the teaching that the Holy Spirit proceeds from the Father alone, grounded in Scripture, patristic witness, and conciliar authority. Drawing on precise theological reasoning, it exposes the logical and doctrinal consequences of introducing a second cause within the Trinity, showing how such a view undermines divine simplicity and unity. The text engages directly with opposing arguments, carefully analysing biblical language and dismantling misinterpretations with clarity and force. It also offers a sweeping appeal to the consensus of the Fathers and the Ecumenical Councils, situating the doctrine within the living tradition of the Church. Written with both intellectual sharpness and pastoral concern, it calls the reader away from innovation and back to the stability of received faith. At once polemical and deeply theological, this treatise remains a powerful witness to the Orthodox understanding of the mystery of the Holy Spirit.