Joannes Veccus, Patriarch of Constantinople, on the Procession of the Spirit. In Leo Allatius, Vol. I. p. 248
Further, Clement the Stromatist, in the various definitions which he framed, that they might guide the man desirous of studying theology in every dogma of religion, defining what spirit is, and how it is called spirit, says: “Spirit is a substance, subtle, immaterial, and which issues forth without form.”
Source: Fragments (New Advent)