I spent some time yesterday still trying to answer this question. I dug out my Summa and my new Tanquerey book, The Spiritual Life. Both of these gentlemen are giant thinkers. I don't know why I ever even attempt to wrestle Aquinas on my own since it was hard enough in college when I had a great Jesuit professor (yes, it is possible) try to distill it down to our feeble level. All I managed to do is fill my head with more questions. I so need a spiritual director.
Anyway, here's more on the topic. I thought I'd let Sister Immaculata speak for herself. She's a good thinker and distiller. From her booklet, A Study of Conscience.
WHAT IS CONSCIENCE? Part One, since it is a bit lengthy
A. God communicates Himself to the substance of the soul, not to the faculties whose proper object is created things. Therefore, it is not natural for the faculties (intellect and will) to experience God. They need a special "mechanism" to adapt them to attain Him, experience Him or grasp Him by knowledge and love. This mechanism is the Baptismal endowment of grace and the infused virtues and gifts of the Holy Spirit.
1. But it is natural for God to be experienced in the substance of the soul because He is there as creator present to all He has made. Every created soul has its roots in the supernatural Being Who created it as does every creature. The Being sustains all created being who participate in varying degrees in His Being. The difference is that some have the power to become aware of it or experience this Being present because of a greater likeness to Him. In his deepest self, the person can detect a Presence deeper than his own self.
2. Scripture calls it the "eye" of God in man:
He put his own light in their hearts (Ecc. 17:8 -- "light", literally "eye").
He filled them with knowledge and understanding and revealed to them good and evil (Ecc. 17:6)
Their ways are always under his eye; they cannot be hidden from his sight (Ecc. 17:15).
His eyes rest continually on their ways (v. 16)
My eyes are upon the faithful of the land that they may dwell with me (Ps. 100).
We have with us this sense that "Someone sees"; Someone within me sees.
Where could I go to escape your spirit? Where cold I flee from your presence (Ps. 139:7).
3. This experience does not mean a clear concept that "God is in me"; the person may not have learned that yet, and it does not mean an experience of burning love such as the mystics describe in the later stages of the development of conscience. It is rather better described as a discernment of peace, well-being and goodness which stems from the unity of one's powers of intellect and will. Both together discern this presence though it is not the proper object of either one. Since it is natural, we may "overlook" it, take it for granted, not become conscious of it until -- it is brought to consciousness by education (formation of conscience) or -- we lose it by acting against it. The soul loses its unity; God does not depart because He must always be there as creator to sustain the person in existence, but His very presence is the pain itself, or rather, the soul cannot endure the anguish of its own disunity; the anguish of having cut off its own life, of having denied its own source and center. As "we miss the water when the well runs dry" so we realize and recognize the peace, security and well-being we had when we experienced how pained we are without it. The presence of the good is there within, still admonishing us to return to the true path.
a. "God who probes the heart awaits him (man) there. There he discerns his proper destiny beneath the eyes of God" (Constitution on the Church in the Modern World [CCMW] 14:3. p. 213). He probes the heart with His clear knowledge.
You probe my heart, examine me at night; you test me and find nothing (Ps 17:3). Yahweh, you probe me and you know me (Ps. 138:1) I, Yahweh, search the heart; I probe the loins (i.e.; the inmost being) (Jer. 17:9-10).
God's "knowing" in him is the very source of the person's recognition of good and evil.
b. This presence "knowing" me is equated with my inmost self:
My inmost self instructs me (Ps. 16:7)
St. Paul calls in the law of my mind (Rom, 7:23). It is "a law in the heart written by God" (CCMW 16, par. 1, p. 213)
The spirit of God is at the very center of the "I".
The spirit Himself and our spirit bear united witness that we are the children of God (Rom. 8:16)
The created spirit can relish an inner harmony with the Creative Presence which is the unmistakable inner testimony that there is unity in the conscience, unity in the person. Thus conscience is the interior personal witness of our own uprightness and conversely of our failure to correspond with the Truth.
Yet even when our conscience does not reproach us, we must always live in humility and a loving reverence of God because our conscience may be in error, more or less, through ignorance. Scripture states that every man needs God's help to recognize the depths of evil into which we are all capable of falling as well as the daily failures we can easily overlook. We need God's action to cleanse us.
But who can detect his own failings, wash out my hidden faults (Ps. 19:12).
And, from St. Paul:
True, my conscience does not reproach me at all, but that does not prove that I am acquitted: the Lord alone is my judge (I Cor. 4:4-5).
4. "Conscience is the most secret core and sanctuary of a man. There he is alone with God whose voice echoes in his depths" (CCMW 16, par. 2, p. 213).
Each man is alone with God. This is the awesome and consoling reality that we are accountable to Him alone. He knows every thought, our struggles, failures and efforts. We do not have to labor for words to explain to Him because He understands. He is the secret sharer of every life. The dread He inspires is not that of terror, unless we have deliberately rejected Him and deserve His wrath. It is rather the fascinating, attracting sense of awe in the presence of Something too tremendous to comprehend.
This aloneness is the mystery of each man's solitude. Since each is a particular image of God, each has a different role in life which no one knows but the man himself, yet only as God gradually reveals it to him through the growth and development of his conscience.
No one can communicate the mystery of his being to another. There, in the "ontological solitude" of his very being, he is alone with God. God alone can know a man completely. That means that each decision is my own, as personal as His call to me; then I am responsible for the person I become through my decisions.
Thus, in the heart of man God awaits the fundamental worship; acknowledgment of His presence by conformity to what His presence prompts us to do or avoid.
The primary sanctuary of God is the heart of man: the worship He seeks is one in spirit and truth, conformity to truth in thought, word and action.
Psalm 96 speaks of worshipping Yahweh in His sacred court (v 8). This is, of course, a reference to the temple of Jerusalem, but this earthly temple is a symbol of His dwelling in the very center of a human being.
More later. Stay tuned for Part II. BTW: The CCMW she cites is a document of Vatican II.