The Inner Light, before Quakerism
As I continued through Paul Tillich's A History of Christian Thought, I ran into this passage during a discussion of thirteenth-century theology:
[The Franciscans said that] All knowledge is in some way rooted in the knowledge of the divine within us. There is a point of identity in our soul, and this point precedes every special act of knowledge. Or, we could say that every act of knowledge—about animals, plants, bodies, astronomy, mathematics—is implicitly religious. A mathematical proposition as well as a medical discovery is religious becuase it is possible only in the power of these ultimate principles which are the uncreated divine light in the human soul.
This is the famous doctrine of the inner light, which was also used by the sectarian movements and by all the mystics during the Middle Ages and the Reformation period, and which in the last analysis underlies even the rationalism of the Enlightenment. The rationalists were all philosophers of the inner light, even though this light later on became cut off from its divine ground. (185)
Tillich goes on to oppose this view to Thomas Aquinas, but it also echoes Tillich's earlier comments about the rationality that follows ecstatic movement. If the experience of that light as something explicitly divine is lost, it's relatively easy to move directly into a rationalist view.
For myself, I've always felt that rationalism was broken, missing key pieces to ground itself. The most I've been willing to let pure rationality do is establish probabilities, which are highly useful but deliberately limited. I've doubted for years that 'knowledge' is something we humans compose out of knowable facts. On reading this section of Tillich's history, I can see both why I've felt that way and why I find Quakerism so amenable. I'm drawn to that light and doubtful when I don't think the light is present. The 'divine ground' seems necessary to me, however hard hyper-rationalists try to keep it out.