Let me give you a few scattered answers, and see if you find any of it helpful.
First, these terms are philosophical. Picking up some introduction to St. Thomas Aquinas and his treatment of intellect, will, etc. would be helpful to you. That being said, realize that there is a difference in the use of these terms in St. Teresa and St. John of the Cross. St. John of the Cross was a very well-educated Friar, and he knew the scholastic terms in great depth. St. Teresa, while her spiritual doctrine is in harmony with St. John, was not formally educated, and so she uses terms such as imagination/intellect or spirit/soul/mind interchangeably. So, if you want to understand these terms in a systematic and precise way, you’d really need to read a bit of scholastic terminology (Aquinas) and stick to St. John of the Cross. However, many people find St. Teresa easier to read because of her honest, attractive style.
All that being said, I won’t define these terms for you, because my philosophy’s a bit rusty, so hopefully somebody may come along and give you the definitions you’d like. But let me try to answer a bit of your last paragraph, the practical stuff.
I want to understand what the imagination’s role is, where it ends and where the role of the intellect takes over. Where the intellect ends and the role of the soul takes over, and where is the will in relationship to the others and how do they all come together to make “me- a person.”
Ok, so the role of the imagination in one’s prayer life really depends on the individual. Following the doctrine of St. Teresa and St. John, in prayer we are to constantly try to remain recollected, aware of Christ’s presence. For some with vivid imaginations, a very lively imagining of a particular Gospel passage may help recollection. Others (St. Teresa includes herself here) may have a very dull imagination, and are better recollected by recalling a passage from a spiritual book, or by looking at a particularly striking holy card. The use of one’s intellect (pondering a particular Gospel scene or truth of our faith) may be God’s way of leading one to deeper prayer at a certain time. At other times, reasoning or imagining may lead us far from recollection, where we end up thinking about what’s for dinner or what we have to do once prayer time is over. So, to ask when the intellect or the imagination or the soul takes over…well, that’s not quite how it works. God will sometimes work through the intellect, sometimes the imagination, sometimes deep in the soul, in a way where one’s senses and intellect have nothing to do with it. St. Teresa talks about advanced stages of contemplation where the memory or the imagination are spinning quite crazily, yet God is working on a deeper level.
The will is our choice, so to speak. Even if my imagination is not working well, I can’t concentrate, I can’t even think much about God, I still have my will. I can give that to God, and say (so to speak) “God, I can’t concentrate on you, I can’t think about you, but I choose to sit here and be with you.” That choice of the will (which you may be constantly renewing in your prayer time, as you keep getting distracted) is most important. St. Teresa says that a soul that perseveres in prayer, that gives its will continually to God without getting discouraged by “lack of results” or “good prayer” will not be lost.
And this leads to an important last point. St. Teresa constantly stresses that the only sign that one is praying is virtue: love of neighbor, detachment from the desire for earthly things, and humility. If one is giving time to God in prayer and is growing in these virtues, he is praying, even if his imagination or intellect don’t seem to be “doing much” in prayer. On the other hand, one may have beautiful thoughts about God, consoling meditations, peaceful hours in prayer. But, if he is impatient when interrupted, uncharitable with others, proud of his “success” in prayer, he is not praying. Virtue alone is the measure of prayer.
I hope that’s somewhat helpful. I am in formation as a Discalced Carmelite Friar, so the spirituality is fresh in my mind, but to be honest the philosophical terms are a bit hazy, so I apologize for not getting to all you wanted to know. May God bless you as you continue this journey toward Him. And stick with these two beautiful Saints and Doctors of prayer, they are a sure way to true Catholic spirituality.
In Christ through Mary,
Frank