On First Principles by theologian Origen. The philosophical text primarily explains, from Origen's point of view, various concepts in Christianity. On First Principles is considered one of the first theological works in the Christian canon. One of the concepts explored in the text is the idea of the Holy Trinity. Origen explains the logic of the Trinity--composed of the Father (God), the Son (Jesus Christ), and the Holy Spirit (the supernatural energy that both Father and Son are made of, or contain, respectively)--positing that the elements are both codependent and independent of one another.

Another concept Origen explains is free will, which he defends. One of the most interesting and controversial theories in On First Principles is the concept of reincarnation, which is a belief found in Hinduism and Buddhism, but is not cited in the Christian scriptures as one of its tenets. The concept or mechanisms of the soul seems to be an overarching theme in On First Principles. For instance, Origen believed that all souls are absorbed by God in the afterlife to form a singular conscious entity.