Arcana Coelestia #8609
8609. 'And they took a stone' means the lowest level of God's truth. This is clear from the meaning of 'a stone' as truth, 643, 1298, 3720, 3769, 3771, 3789, 3798, 6426, in this instance the lowest level of truth since it was put underneath him and he sat on it. An ordinary stone furthermore means a lower level of truth, while a precious stone means a higher or more internal level of truth. What the lowest level of truth is becomes clear from what has been stated above in 8603 about the levels of truth that follow one another in order; that is to say, there are interior truths and exterior truths, the lowest of which are meant by the lowest level of truth.
Leviticus 14:6-7
6
As for the living bird, he shall take it, and the cedar wood, and the scarlet, and the hyssop, and shall dip them and the living bird in the blood of the bird that was killed over the running water.
7
He shall sprinkle on him who is to be cleansed from the leprosy seven times, and shall pronounce him clean, and shall let the living bird go into the open field.