The immaculate conception refers to the dogma that Mary was conceived in the womb of her mother, Ann, without Original Sin. The Scriptural evidence for this dogma is, of course, the angel's greeting: "Hail, full of grace" (Lk 1: 28). Mary is full of grace, both intensively and extensively, that is, at the moment of the greeting, and her fullness of grace extends back to the very moment of her conception - otherwise she would not be truly "full of grace".
But imagine what it would mean to be full of grace. This means that Mary did not "know" sin. She had no experience of sin, unlike the first Eve, who wanted to know what it would be like to become "wise", to taste from the tree of the knowledge of good and evil. The first Eve wanted knowledge, that is, she wanted to taste independence from God. She was tempted to enjoy the pseudo sophistication of being her own god, and that is what she chose - her husband, who was with her, also made the same choice.
Mary, on the contrary, had no knowledge of this sort. And because she was without this knowledge, she possessed another kind of knowledge, one that is outside our capacity to acquire. What would it be like to live and experience the world without a knowledge of sin? I have no idea, and I will never know that, but her disdain for a certain knowledge begat a knowledge, while the desire and the decision to "know" sin darkened the minds of the first parents of the human race.
Original sin was fundamentally about choosing to be more than what we are. There are very real limits to the human condition. We are limited intellectually by matter, which means we are limited by geography (place), history (time), and sense perception. We are radically dependent upon others, and most importantly, we are radically dependent upon God. The tree of the knowledge of good and evil represents independence, as a tree towers high above and stands independently, apparently sufficient unto itself.
If Mary had no experience of sin, then she did not desire to be more than what she is. She knew her human limitations and accepted them. We, on the contrary, are familiar with chaos and disorder because we sin, and to sin is to reject our own limitations, to reject our status as "child" dependent upon God, who is the measure of what is true and good. The irony is that when we try to transcend our limitations, we walk away with much less, but Mary walks away with more, because she lived in harmony with the limits and rhythms of creation.
To get some idea of what it must be like to know a life without sin, try to remember what it was like to experience the world as a child. Sometimes I do get glimpses of a knowledge I once had, a memory of an experience of the world that was mine as a child. It was a very rich experience, far beyond what I am capable of expressing in words because I am simply not a skilled poet. We saw the world with the eyes of innocence, without desire, without a certain degree of concupiscence, but that semi-paradisal experience gradually faded into the distance as our lives become less innocent. As the knowledge and taste of sin increases, our capacity to know this world from the angle of innocence, in all its richness, is dulled. There are so many epistemic conditions that are necessary for certain kinds of knowing, and many of them have to do with character.
People of radically different character find it very difficult, if not impossible, to understand one another. Mary possessed certain epistemic conditions that we have lost through our lack of innocence. There is no doubt that certain kinds of knowledge were lacking to her, i.e., a knowledge of chemistry, history, statistics and physics, but all knowledge of creation serves the very end and purpose of creation, which is the sabbath worship, as we read in the first chapter of Genesis. And so, there is nothing that these other areas of knowledge could provide, in terms of contemplative joy, that Mary did not already possess as a result of her complete innocence.
But although Mary would have experienced a peace and a knowledge that is outside our reach, she also suffered more than any of us could, because her love of God is exceedingly greater than ours. She did not know sin from within, but she knew sin from without. She was living in a sinful world, surrounded by indifference to holiness. Sin would have "rubbed her the wrong way", since it was so contrary to what she is.
And so from one angle, living in this world would have been far more difficult for her than for any of us - the holier a person is, that is, the greater a person's love for God and neighbour, the greater is the pain of having to witness sin against God and neighbour. She had a deeper share in the life of her son than any of us, and so her peace was greater, her knowledge far exceeds our own, but her suffering at the same time far exceeds anything we are capable of understanding. Her suffering was not a joyless suffering, but a peaceful, joyful and enlightened suffering.