Head
The human head is a symbol of consciousness and thought (“the seat of heavenly light”; alchemists referred to themselves as “children of the golden head,” etc.). The wise giant Mimir, whom Odin consults, is a head without a body. Foolish or uncontrolled behavior is called “headless.” (Traditionally, the heart has been associated with reason instead, a notion sometimes reflected in dreams.) The head is "high up," the highest part of the body, closest to the heavens, and thus can share this symbolic value.
The language of symbols particularly emphasizes the round shape of the head, which has created associations with vessels, the microcosm, and also the universal mythological ideas of the head as the sun and/or moon, and vice versa.
The Self and Identity
The head also represents our self and our identity (compare headgear). When a person is baptized, water is poured over the head to signify complete transformation.
As mentioned, the head represents consciousness and identity. Decapitation can thus symbolize getting rid of intellect or a particular mindset. The rest of the body is, so to speak, instinctual.
The Monster and Identity
If a human's body transforms into that of an animal while the head remains unchanged, one might assume that the person is mentally still him or herself. However, if a human's head transforms into that of an animal while the body remains human, one might assume that the creature is something entirely different. Compare, for instance, a centaur and a minotaur—both are part animal and part human, but the former is human-like, while the latter is a monster.
The Headless
Those who lack a head are forever condemned, like the Headless Horseman or the once-popular image of a ghost carrying its head under its arm.
The Bodiless
Heads without bodies were, in various cultures, considered a type of ghost, manifesting the spirits of the dead. The essence of the deceased resides in the skull.
A bodiless head can suggest a detached intellect—a mind without instincts.
The Microcosm
The head is round and contains an inner world—a microcosm reflecting the outer universe. It is a symbol of wholeness and transformation, considered sacred in most cultures and, in alchemy, the vessel for transformation.
Omniscience
The head, therefore, is an image of totality, which is likely why Mimir is merely the round, all-knowing vessel and nothing else. It is the same vessel Hamlet seeks answers from in vain, as does Faust. Additionally, there are several accounts from ancient Mediterranean history of magical practices using actual severed human heads as oracles.
One could argue that the head was considered the seat of the soul, the "aeon," or what we might today call the collective unconscious, while consciousness was more associated with breath, lungs, and the heart. (See "Transformation Symbolism of the Mass", par. 373.) This might explain why the "skull" was consulted for answers otherwise unknown.
Alchemy and Decapitation
For alchemists, the head was a symbol of the substance of mystical transformation. In their imagery, decapitation could symbolize the acquisition of this. An Arab alchemist noted that the head resembles a "distillation apparatus."
According to alchemical texts, God created the skull to capture the soul. The head is sometimes synonymous with or equated to the work's vessel, and vice versa. Zosimos refers to "the children of the golden head" as a term for "the philosophers" and frequently mentions "the wisest of stones, found in the head." ("Transformation Symbolism in the Mass", par. 366.) Jung interpreted this as implying that the alchemist's process was fundamentally psychic and projected onto matter.
In line with this, decapitation can symbolize a separation from "‘the great suffering and sorrow’ that nature causes the soul" (through the body)—in other words, liberation (Mysterium Coniunctionis, par. 730). A psychological equivalent would be the therapeutic process of "separating" the ego from affects and instincts, viewing them objectively rather than being one with them. This is the first necessary step of three toward coniunctio. (Ibid., par. 671.)
Another perspective on the adept's decapitation is the severing of intellect from instincts—achieving an "objective standpoint" in relation to oneself or one’s unconscious. Conversely, it could mean its opposite: sacrificing intellect; relinquishing intellectualizing to experience the mystery directly (Redemption Motifs in Fairy Tales, p. 117).
The Sacred
In many cultures, the head is particularly sacred as the seat of the human spirit. In some traditions, the head is protected from higher powers, while in others, care is taken to ensure that nothing comes between the head and the great spirit above. (See also FACE.)
The Brain
The brain, as the content of the head, inherits much of its symbolism—awareness, intelligence, and so on—but the substance itself has its own symbolic quality. To the alchemists, the brain was synonymous with "the secret substance," the seat of spirit/water, and they referred to "the brain-stone" ("which is not a stone")—compare Dragon. Furthermore, the "white" brain was associated with the moon, the mystery of baptism, but also with "the infernal gods," likely an allegory for consciousness in its criminal aspect (compare Lucifer), that is, as a deviation from the natural. (Mysterium Coniunctionis, par. 626.)