Synthesized answer
The provided passages do not contain enough information to answer the question about the central thesis of "Über das Organ der Seele" by Samuel Thomas von Soemmerring.
The only passage available is the title and category information, which indicates the work is about the brain [110]. This is insufficient to determine the author's central argument or thesis.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Über das Organ der Seele by Samuel Thomas von Soemmerring Categories: Brain Pages: 110