John Damascene:
The Word must also possess Spirit [the Greek term Πνεῦμα denotes both “breath” and “spirit”].
For in fact even our word is not destitute of spirit; but in our case the spirit is something different from our essence.
For there is an attraction and movement of the air which is drawn in and poured forth that the body may be sustained.
And it is this which in the moment of utterance becomes the articulate word, revealing in itself the force of the word.
But in the case of the divine nature, which is simple and uncompound, we must confess in all piety that there exists a Spirit of God, for the Word is not more imperfect than our own word.
Now we cannot, in piety, consider the Spirit to be something foreign that gains admission into God from without, as is the case with compound natures like us.
When we heard of the Word of God, we considered it to be not without subsistence, nor the product of learning, nor the mere utterance of voice, nor as passing into the air and perishing, but as being essentially subsisting, endowed with free volition, and energy, and omnipotence.
So also, when we have learnt about the Spirit of God, we contemplate it as the companion of the Word and the revealer of His energy, and not as mere breath without subsistence.
For to conceive of the Spirit that dwells in God as after the likeness of our own spirit, would be to drag down the greatness of the divine nature to the lowest depths of degradation.
But we must contemplate it as an essential power, existing in its own proper and peculiar subsistence, proceeding from the Father and resting in the Word, and shewing forth the Word,
neither capable of disjunction from God in Whom it exists, and the Word Whose companion it is, nor poured forth to vanish into nothingness,
but being in subsistence in the likeness of the Word, endowed with life, free volition, independent movement, energy, ever willing that which is good, and having power to keep pace with the will in all its decrees, having no beginning and no end.
For never was the Father at any time lacking in the Word, nor the Word in the Spirit.
John Damascene (c.675-749): De Fide Orthodoxa 1, 7.
