A hard problem of explanation arises from the following starting points:

1. Reality is fully explicible.
2. Reality is partly contigent.
3. If reality were fully contigent, then it would be inexplicible.
4. Hence, reality is partly necessary.
5. Necessary reality cannot be explained in terms of contigent reality.
6. Contigent reality cannot be ultimately explained in terms of contigent reality.
7. How then can reality be explained?

Now, perhaps contigent reality can be explained in terms of necessary reality which in turn can be explained in terms of (parts of) itself. But how exactly can contigent reality arise from necessary reality and how exactly can necessary be explained in terms of itself?