Thanks everyone for your comments and thoughts. It's interesting to get others' perspectives. So here goes my take on the "Inner Light" issue.
I think this is one of Quakerism's most unique, most orthodox Christian, most unscriptural and most biblical doctrines of them all! As many of you pointed out in comments to my last post, the image of the Inner Light comes directly from the Bible, from John and I John. John 1 talks about the Word being God, Light, and Life, and says this being who is all of these things became flesh in Jesus. Later in John Jesus says, "I am the Light of the world." I John says that if a person is not in the light they are in the darkness, and many other places in the New Testament and the Hebrew Scriptures use the metaphor of light and darkness to describe spiritual states of awareness. Those who follow God are a city on a hill (at night) that cannot be hidden, a lamp that shouldn't be hidden but should be put on a stand to give light to all in the house (Matthew 5:14-16) or all who come in (Luke 11:33-35). There are many instances where spiritual lack of awareness is expressed as blindness, indicating that light is not being allowed to come into the person (as through the eyes).
It seems that in the New Testament, light refers to God or Jesus, and the truth of the ability of humans to know God, be aware of God, and be obedient to the call of God in their life, especially through Jesus' command to love. So this idea of the Inner Light is in many ways very biblical, if we think of the Inner Light as God, or in Quaker-speak, "that of God in every person."
But "that of God in every person" is perhaps where we get a little off track from mainstream Christianity--and I'm not saying that's a bad thing here. It is, however, difficult to see where we got this idea of that of God being in each person, except from Genesis 1:26, where God says, "Let us make humankind in our image." This is what got me thinking about the Inner Light to begin with, because last week in my systematic theology class we discussed what it means that humans are created in the image of God.
The first thing that comes into my head when I think of being created in the image of God is that we all have the Inner Light in us--a piece of God that recognizes and longs for God. Calvin criticizes this idea (although Quakers hadn't come along yet) by speaking of some who "thought the soul to be a derivative of God's substance, as if some portion of immeasurable divinity had flowed into man" (John Calvin, Institutes of the Divine Religion I.XV.5). I don't think this is the original idea of the Inner Light that Quakers had--I don't think they meant that a piece of God is possessed by each person and is somehow separate from God and only in that person. I think the original Quakers who used this term meant that God is at work in each person, and each one has the ability to know God (as opposed to the doctrine of predestination).
Today, however, I'm not sure if we always remember to make this distinction. It's all too easy to think of the Inner Light as something we own, or something that is me rather than being something of God that is communicating with me. It's hard to know where to draw that line, because if I'm created in the image of God, there is something about me that is similar to God, that connects me to God, but that doesn't mean any part of me is God.
It's nice to think of little pieces of God being spread out through all humanity--like R.W. Emerson's "over-soul." He was influenced by Quakers, and he thought of "that of God in every person" as sort of a pre-Jungian collective unconscious, the part of ourselves that connects with other people, but he didn't really believe in a God who had a consciousness separate from humanity.
But I think that takes away the whole point of the Inner Light. To me the Inner Light is that part of me that calls out to me that there is something (Someone) Other, something not-me, and this not-me affirms my humanity and my goodness, as well as calling me to let go of the parts of me that aren't good. This Light calls me to give up the security of the darkness where I can hide, and calls me out into the glorious and sometimes painful Light where everything is exposed and laid bare--but where everything is loved. In the Light I can be who I am and be challenged to step more fully into who I am created to be every day, taking baby steps away from the darkness and toward the Light.
I too like the image of Light because it can't be defined, even by science. Is it a wave, or a particle? Who knows! Both. But how can it be both? It's a paradox, but we can see what it does, we can all recognize that it's there and we know its effects. Without it there would be no life on this planet, as there would be none without the ultimate Light, who is God, with whom we can participate and commune as a tree growing by streams of water (Psalm 1) in the sunshine.