On the Holy Spirit is a theological treatise by Basil the Great, bishop of Caesarea and one of the Cappadocian Fathers. It presents a lucid and edifying appeal to Scripture and early Christian tradition in order to prove the divinity of the Holy Spirit. St. Basil forms clear and careful arguments against heresy of the early Christian church, mainly the second wave of Arianism. Basil proves that the Holy Spirit and the Son are not, by any margin, lesser in divinity than the Father, placing his defense of the doctrine of the Holy Spirit within the tradition of the Church.