This impersonal layer of the psyche I have termed the COLLECTIVE UNCONSCIOUS-"collective" because it is not an individual acquisition but is rather the functioning of the inherited brain structure, which in its broad outlines is the same in all human beings, and in certain respects the same even in mammals. The inherited brain is the product of our ancestral life. It consists of the structural deposits or equivalents of psychic activities which were repeated innumerable times in the life of our ancestors. Conversely, it is at the same time the ever-existing a priori type and author of the corresponding activity. Far be it from me to decide which came first, the hen or the egg. [208] Our individual consciousness is a superstructure based on the collective unconscious, of whose existence it is normally quite unaware. The collective unconscious influences our dreams only occasionally, and whenever this happens, it produces strange and marvellous dreams remarkable for their beauty, or their demoniacal horror, or for their enigmatic wisdom-"big dreams," as certain primitives call them.
— Carl Gustav Jung
Jung reaches for biology here — the inherited brain, structural deposits, ancestors — because he needs you to understand that what surfaces in the big dream does not belong to you personally. This is not your unprocessed day, not your mother's face, not the residue of yesterday's anxiety. It is older than your name. The collective unconscious does not care about your biography; it carries the grammar of a species, and occasionally that grammar speaks directly through the dreaming mind.
The hen-or-the-egg aside is worth pausing on, because Jung is doing something careful in it. He refuses to say whether the psychic activity created the brain structure or the brain structure generated the activity — and that refusal is the point. Depth psychology does not need a materialist foundation. The "a priori type" is both cause and effect, pattern and origin, and the individual consciousness built on top of it proceeds in almost total ignorance of what it rests on.
What the big dream announces, then, is not revelation from above but pressure from below — the ancient substrate asserting itself, briefly, in the language of beauty or dread or enigmatic precision. The "primitives" Jung cites understood this experientially before the vocabulary existed. What made the dream big was not its interpretability but its weight: the sensation of something speaking that was not, in any ordinary sense, you.
Carl Gustav Jung·The Development of Personality·1954