Dear Mitchell.
Thanks very much for taking the time to put this together.
I hope you won't think that I'm being unduly difficult or stubborn, but I'm still not convinced by the quotes you offer, even though I understand that you think them persuasive.
Here's a general point. You say that you are highlighting in Bold words you consider equivalent to 'biological'. I note, therefore, that you have highlighted the word 'psyche' several times. Yet you offer no explanation as to why you think this, so I assume you think that the connection is obvious and requires no explanation. This leads me once again to wonder exactly how you define the term 'psyche'; you haven't yet done so, although I did ask.
You note that Jung preferred the term 'psyche' to 'mind', I would say for good reason. Yet here, for example, you make no distinction: you interpret “the psyche contains all the images that have ever given rise to myths”, as “myths and religion derive from the brain/mind”. You also quote Jung: "By psyche I understand the totality of all psychic processes, conscious as well as unconscious", and interpret this “i.e. in the human mind”. So one point would be that you seem to be ignoring what Jung is saying; you equate psyche and mind, even though he finds 'mind' inadequate to describe what he means. A second point is that you haven't defined either 'psyche' or 'mind'. I remember Jung's foremost disciple Marie-Louise von Franz saying somewhere that many people confuse the psyche with the brain – I believe her exact phrase was closer to “many people project the psyche onto the brain”. (I regret I can't offer you the source of the quote; it was a long time ago that I read it.) So I'm wondering whether you are one of these people she is referring to. I would be interested in any quotes you can offer where Jung clearly and unequivocally equates psyche with brain or mind.
It is vitally important in these debates that we define precisely our terms. The quote from Jung above is almost a tautology, so it isn't very helpful in knowing what he meant by 'psyche'. It is therefore crucial to understand what he means by 'all psychic processes'. I'm guessing that for me this is far more extensive than for you. As I'm sure you know, Jung was much influenced by the ancients: Gnosticism, the Greeks. On this point I'm reminded of a fragment of Heraclitus: “The limits of the soul you would not find out, though you should traverse every way”. Thus the soul (psyche?) is virtually infinite, which is one way of looking at it. Here's another interesting definition: psyche (n.), "animating spirit," from Latin psyche, from Greek psykhē "the soul, mind, spirit; life, one's life, the invisible animating principle or entity which occupies and directs the physical body; understanding, the mind (as the seat of thought), faculty of reason" (https://www.etymonline.com/word/psyche). Here the first half seems closer to my worldview (and possibly to Jung's?), the second half closer to you. And I would add that in these definitions psyche would seem to have too many diverse interpretations to be a useful term; more precision is needed.
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I'll now comment on some of your quotes.
“The distinction between mind and body is an artificial dichotomy…So intimate is the intermingling of bodily and psychic traits that not only can we draw far-reaching inferences as to the constitution of the psyche from the constitution of the body, but we can infer from psychic peculiarities the corresponding bodily characteristics."
This sounds like something from Jung's later interpretation of 'archetype' after he coined the term 'psychoid'. My first point would be that, even though Jung says that the intermingling of bodily and psychic traits is intimate (indeed it is, as in similar fashion consciousness and the brain are intimately intermingled), he nevertheless distinguishes between them. This reminds me of the work Jung did with Wolfgang Pauli, concluding that there was an apparent difference between psyche and matter, but they both emerged from a hidden source where they were more equivalent, i.e. somewhere beyond 'nature'. In effect that they were both manifestations of the transcendent archetypes (blueprints). It is obviously only my interpretation, but I think that here Jung is saying something similar to the esoteric idea of 'as above, so below', that each level of reality reflects the higher ones; the body reflects the psyche, which reflects the archetypal level. I think that Jung frequently said that archetypes cannot be perceived directly, but can only be inferred from their influence upon the material world. I would deduce from such statements that he did not think that they were 'biological' or 'natural'. He says that in so many words on the next page following your quote: “The psyche is still a foreign, almost unexplored country of which we have only indirect knowledge”. I don't think that he would say that if he thought the psyche was as as natural and biological as you seem to be suggesting.
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"The ruling world religions... contain a revealed knowledge that was originally hidden, and they set forth the secrets of the soul in glorious images." I would need some clarification on how you consider the soul and its secrets to be natural or biological.
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"Instincts are given in the case of every newborn individual and belong to the inalienable stock of those qualities which characterize a species. What psychology designates as archetype is really a particular, frequently occurring, formal aspect of instinct, and is just as much an a priori factor as the latter." [mythological motifs/ archetypes are innate/inherited/biological]
A minor quibble would be that your quote talks of instincts, whereas your interpretation says instead mythological motifs/archetypes. These are surely not directly comparable. I refer again to Jung's development of the term 'psychoid', to include both psyche and matter (including instincts). I also refer again to my problem with your failure to define the word 'psyche'. You seem to be assuming that anything inherited is biological. If mythological motifs are inherited, then they could be inherited psychically, which would not be the same thing as biological inheritance. (I'm especially interested in the biologist Rupert Sheldrake's theory of morphogenetic fields, which may have some indirect relevance here.)
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"The symbols of mythology are not manufactured; they cannot be ordered, invented, or permanently suppressed. They are spontaneous productions of the psyche, and each bears within it, undamaged, the germ power of its source." [You then say: Myths come inborn from the mind.]
Again use of 'mind' when Campbell says 'psyche'. Do you not make any distinction? Is there some confusion? And what does he mean by 'its source'? Possibly something beyond the psyche? That's what the quote seems to suggest.
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"Wherever the poetry of myth is interpreted as biography, history, or science, it is killed. The living images become only remote facts of a distant time or sky. Furthermore, it is never difficult to demonstrate that as science and history, mythology is absurd. When a civilization begins to reinterpret its mythology in this way, the life goes out of it, temples become museums, and the link between the two perspectives is dissolved".
I don't see what your point is here, or how it helps your argument. If myths were merely science or history, then they would be far closer to being 'natural' or 'biological'. Campbell saying that this interpretation kills them suggests that they are something beyond this.
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"A mythology may be understood as an organization of metaphorical figures connotative of state of mind that are not finally of this or that location or historical period, even though the figures themselves seem on their surface to suggest such a concrete localization. The metaphorical languages of both mythology and metaphysics are not denotative of actual worlds or gods, but rather connote levels and entities within the person touched by them. Metaphors only seem to describe the outer world of time and place. Their real universe is the spiritual realm of the inner life. The Kingdom of God is within you."
If myths are not of a particular location or historical period, again this suggests that they come from a place beyond space and time. As your quote suggests, metaphors only seem to describe the outer world of time and place. They actually refer to somewhere beyond; where would that be?
Is the 'spiritual realm' natural or biological? You must have an unusual understanding of the word 'spiritual'.
The Kingdom of God is indeed within us, but surely this refers to the spiritual state that consciousness can achieve through spiritual practice, known by such terms as 'enlightenment', 'nirvana', i.e. rising above nature and biology, the physical world.
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Jung and Campbell may say that “humans have an instinctual need for mythology and hence, religion. It's embedded in us and can't be thrown aside”. This sounds, however, as though this psychological need may be illusory (to most people, although perhaps not to you); it does not address the issue of whether the beliefs are true. In the phrase “the inward journey” I again assume that by 'inward' you understand nothing more than the natural or biological. I would argue, and I think Jung would agree, that what is within goes far beyond that.
I regret, therefore, that I remain unconvinced by your argument, and have found nothing in these quotes to persuade me to change my mind. I am fascinated that someone interested in Darwin and evolutionary biology should also be interested in Jung and Campbell; I imagine that you are something of a rare breed. I would say, however, that you have chosen to interpret Jung and Campbell in ways which conform to your worldview, whatever that is – and of course you will say that I have done the same. I would respond that, at the very least, other interpretations to yours are possible. I will therefore indeed “continue to fight for a divine source of myths being external”, and will continue to embrace Jung and Campbell from that perspective. If you can clarify your thinking on my objections here, I'll consider having a look at your book. In advance of that, I would agree that all humans have religious inclinations, and that religion is a universal feature of humanity, that both Jung and Campbell think this, and wouldn't therefore need to read it to be persuaded of this.