"Do not, like the pupils of teachers, over-scrutinize the words that are written from experience for the fostering of your way of life, which render your soul great because of the greatness of the insights found within them" (Homily one of St. Isaac the Syrian Ascetical Homilies).
One of the problems when speaking of the spiritual life is there are no fixed words or categories. Spiritual writers will use words such as soul, heart, mind and spirit in slightly different ways. Our tendency is to try to understand spiritual writing in the same way we would understand an academic text--"like pupils of teachers." We define terms and map out logical implications.
However, St. Isaac warns us that spiritual writing is not understood this way. We must "discern the purport of all the passages that [we] come upon in sacred writings," he says. Discernment is a different faculty from the rational, analytic part of the mind. Discernment is much more intuitive, more reflective, more platonic. By platonic I mean that the understanding comes not from figuring out what is presented but from recognizing the likeness of what has been presented to something that already exists in yourself. Exact wording or carefully defined terms do not help here. What is necessary is self knowledge that recognizes the likeness (image or pattern) of what is presented as something already known (albeit vaguely) in yourself. But in recognizing this likeness, what is presented becomes a kind of illumination of what is already known (now more clearly).
Lanza Del Vasto, a French Christian philosopher of the mid 20th century said the same thing in his writings. He said that if what he was saying is true, the hearer will recognize it as something that he had always believed. This, I think, is part of what illumination means. Illumination is the seeing more clearly of what you have already known. This is why, in the Christian tradition, the words of a spiritual father or mother are so important. Illumination comes from spiritual words--not from analyzing them, but from discerning them as true words because they already exist deep inside you in your heart, in your true self. Those of us who live in the spiritual desert, however, have very few and very weak spiritual fathers and mothers*, so most of our encounters with spiritual words comes through reading spiritual writings. But just as Jesus said to his hearers, "Be careful how you hear," so we who read must be careful how we read. We must read with discernment (and humility!) not expecting to gain spiritual benefit from rational analysis. And we must hold what we see lightly (this is the humility part) knowing that if we see at all we "see in a glass darkly."
*Please note that a weak spiritual father is much, much better than no spiritual father. When I say weak here, I mean weak by the standards of the saints of the Church. I do not mean weak in relation to us, the spiritual children. Nevertheless, many have made the observation that the parents of a teenager know almost nothing, but in the years that the young are away at college, the parents sure seem to learn a great deal. As we grow, our appreciation for our spiritual fathers and mothers generally grows too.