Gregory of Nyssa

When he was asking how to give a name to what cannot be grasped in thought and did not discover a word expressing an interpretation of the incomprehensible, he called "glory" and "substance" whatever underlies all good and is not sufficiently known or spoken of. The underlying essence of being he dismissed as unnamable. However, interpreting the unity and inseparability of the Son and the Father, and the Son's being contemplated indefinably and invisibly with the indefinable and unseen Father, he addressed him as "radiance of glory" and "image of substance," indicating the unity of their nature by the word radiance and their equality by the word image. For, in connection with a radiant nature, there is no middle point in a beam of light, nor is there an inferior part of an image in connection with a substance determined by it. The observer of the radiant nature will know the radiance in its entirety, and the person comprehending the size of the substance measures it in its entirety with its accompanying image.

The Logos "upholds the universe by his word of power" from nonexistence to existence. For all things, as many as exist in connection with matter and as many as have received an immaterial nature, have one cause of their substance: the Word of unspeakable power.