Thanks for your reply Felix. I was also reminded of the id, ego, and superego, and it is noteworthy how closely the layers of the brain fit to that model. But Freud may well have been influenced by the structure of the brain in developing that model; he studied medicine, which surely included the anatomy of the brian, he was also a pioneer in the study of neurology, particularly in the comparison of human and non-human brain tissues. The relationship between the brain and the mind was understood during his time. I don’t know for sure that the gross morphology of the brain informed his model of the mind, but I don’t think it’s a given that we can take his model out of the context of his own training and research.
For the tripartite divinities, I’m not sure how to evaluate your claims. There are certainly many examples of trinities, but they are by no means universal, and I question whether they even account for the majority of religious belief. And even the claims that certain trinities are the divine trinity for a given religion are not easy to answer. For example, the Hindu deities you mention are viewed as a trinity by some Hindus, but not by all. And it’s been theorized that the Hindu and Western trinities are derived from the same pre-Indo-European culture, where a three-way divides can be found in religion, culture, and politics.
But I think your examples of Taoism, Islam, and Judaism point to a central issue in the project of identifying divine trinities. Take Taosim: the way you’re counting yin, yang, and yin+yang as three should entail that you count father, son, holy spirit, and father+son+holy spirit as four instead of three. There are trinities in Taosim, but important divisions are dualities (yin and yang, tao and te), and the trinities are not always so central (e.g. the eight trigrams, which falls out of two possible states of three attributes (2^3=8) . For Islam and Judaism, the choice of three central things seems arbitrary. Both religions are predominantly unitary, focusing on a unitary god above all else. Counting Allah, Muhammad, and the Koran seems arbitrary: why not the Hadith? If we say that the Hadith just is Muhammad, then shouldn’t the Koran just be Allah? And what of the other great prophets recognized Islam, Abraham, Moses, and Jesus? What of the Five Pillars of Islam? So too with Judaism: again we can arbitrarily cut it off with Yahweh, the Law, and the Prophets, but what of the Messiah whose coming is prophesied? Why count the law and the prophets separately, where both are often considered part of the Torah? And if we’re OK separating the Torah, why not separate it all the way down to the five books of the Pentateuch, and the many other chapters of the Tanakh? Judaism is rich in numerology, but three doesn’t seem particularly emphasized; we can find trinities, but we can also find many other division of the world that aren’t triune. To say that Judaism is tripartite seems ad hoc.
I guess this is all just to say that three has some properties that make it stand out without needing to appeal to brain structure. Any time we have two things, we can get to three by counting, “thing one, thing two, the relationship between them”. That doesn’t seem to depend on the shape of our brains.