Three kinds of magnetism
Why “magnetism?” The unknown is always explained in terms of (on analogy with) the known. Back in Mesmer’s day, the term magnetism had grown to encompass the fields of mineral magnetism, planetary magnetism, and cosmic magnetism. The first was the familiar effect we can experience playing with lodestones. The second we experience watching a compass. The third refers to the space-filling aether originally posited by Descartes and which ended up providing a mechanical basis to explain the propagation of light (luminiferous aether) and gravity (mechanical gravitational aether) along with mineral magnetism.
Mesmer chose his term animal magnetism to clearly distinguish his variant of magnetic force from these others. By animal he meant vital, spiritual, and mental. The universe is pervaded by an invisible vital fluid, and this fluid courses through channels in our bodies and keeps us alive.
Spirit: Latin spiritus “a breathing (respiration, and of the wind), breath; breath of a god,” hence “inspiration; breath of life,” hence “life;” also “disposition, character; high spirit, vigor, courage; pride, arrogance,” related to spirare “to breathe,” perhaps from PIE *(s)peis- “to blow.”
Psyche: Latin psyche, from Greek psykhe “the soul, mind, spirit; breath; life, one’s life, the invisible animating principle or entity which occupies and directs the physical body; understanding” (personified as Psykhe, the beloved of Eros), akin to psykhein “to blow, cool,” from PIE root *bhes- “to blow, to breathe.”
Breath, one and all! Why? Because when people die, they stop breathing. To a cave person, this indicates that grandma’s self (her personality, behavior, proclivities—her spirit) was her breath. When breathing stops, grandma’s self stops. Breath is the original metaphor of consciousness and life.