By St. John of Damascus
The soul, accordingly, is a living essence, simple, incorporeal, invisible in its proper nature to bodily eyes, immortal, reasoning and intelligent, formless, making use of an organized body, and being the source of its powers of life, and growth, and sensation, and generation, nous being but its purest part and not in any wise alien to it; (for as the eye to the body, so is the nous to the soul); further it enjoys freedom and volition and energy, and is mutable, that is, it is given to change, because it is created.
All these qualities according to nature it has received of the grace of the Creator, of which grace it has received both its being and this particular kind of nature.
From An Exposition of the Orthodox Faith (Bk. 2, Ch. 12).