What is the soul made up of?

All material, be it spiritual or physical, is transparent to spiritual vision. That is, it can be seen through.

Each particle of physical material is sufficiently separated from each and every other particle of physical material that to the perception of those spirits that have attained to the other side, physical material, as a solid does not exist, but is seen as a cluster or number of physical material particles or atoms that are held more or less closely together by the law of affinity and the law of vibratory activity. And between each of these different particles of physical material, there exists a space that is sufficiently large for a spirit to see or to pass through.

Fragments of Spiritual Knowledge Pertaining to the Spiritual World