Question: "What is the soul?"
The word for soul in Hebrew is "nephesh," which on its most basic level means "life": "And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul (Genesis 2:7). In this sense, any living creature has a soul (as seen by the use of the word nephesh in Genesis 1:20, 1:24, and 1:30). The word is also used to refer to a person (e.g "All the souls that came with Jacob into Egypt, which came out of his loins, besides Jacob's sons' wives, all the souls were threescore and six (Genesis 46:26)). The soul refers to the internal life of a person ("...my soul is troubled greatly" (Psalm 6:3). The soul also refers to that which leaves the body at death ("And it came to pass, as her soul was departing, (for she died)...(Genesis 35:18)), or in the case of the child brought back to life by Elijah, that which returns to the body ("And the Lord heard the voice of Elijah; and the soul of the child came into him again, and he revived" (1 Kings 17:22). When the person dies, the soul continues to exist, but the souls of the righteous are delivered from the grave: "Yet God shall redeem my soul out of the hand of hades, when he receiveth me" (Psalm 48[49]:15).
St. John of Damascus describes the soul as follows: "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 organised body, and being the source of its powers of life, and growth, and sensation, and generation, mind being but its purest part and not in any wise alien to it; (for as the eye to the body, so is the mind 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" (On the Orthodox Faith, 2:12).
In commenting on Genesis 2;7 and 1 Corinthians 15:45, St. Gregory Palamas says that "living soul" means "ever-living, immortal, which is to say intelligent, for the immortal is intelligent; and not only that, but also divinely blessed with Grace." (On the Holy Spirit 2:8, quoted in Orthodox Psychotherapy; The Science of the Fathers, by Metropolitan Hierotheos (Vlachos), trans. Esther Williams, (Birth of the Theotokos Monastery: Levadia, Greece, 1994), p. 101).
St. Maximus the Confessor says that the soul has three powers: "a) that of nourishment and growth, b) that of imagination and instinct, and c) that of intelligence and intellection [understanding]". Plants have the first power. Animals have the first and second. Men have all three (Metropolitan Hierotheos (Vlachos), p. 106).
When we speak of our "mind," "heart," "reason," and "spirit," these are all aspects of the human soul. As fallen creatures, our soul is in need of redemption, cleansing, and healing.
The Greek word for soul is "Psyche", which is where we get the word "psychology" (the study of the soul). There is a very good book on the Orthodox understanding of the Soul, which is cited above: Orthodox Psychotherapy: The Science of the Fathers," by Metropolitan Hierotheos (Vlachos). However, this title is misleading to English speakers, because when we think of "Psychotherapy," we think of Sigmund Freud taking to someone on a couch, about their mother. But the word here is used in its original sense -- the healing of the soul.
