Signs and Symbols - Their Design and Meaning
Read on page 25. The word concrete is used in context there and if you read the whole page you should get a sense of what it means to an occultist who is steeped in the world of symbolism.
As above so below, and as below so above, to the carrying out of the unity of things.
–Emerald tablet of Hermes Trismegistus known to the Ancient Egyptians as Thoth.
Get it from here:
https://anonfiles.com/q6M92dl6bb/Frutiger_Adrian_Signs_and_Symbols_Their_Design_and_Meaning_pdf
Actually, the whole 360 page book is worth reading.