Sorcery

Sorcery
Sorcery - the short version by Professor Latimer

Most of this information was gained by speaking to my good friend, The Evening Sorceress. It may or may not apply to other sorcerers. Anyone may use that name, after all

Sorcerers like my friend are born with a brain variation that leaves them without certain filters other people have, and so their senses are flooded by chaotic information that is often redundant or useless. They can "sense the spirit world", but that is often overwhelming rather than helpful. (The Evening Sorceress has described this as a "chaos" that constantly assaults her senses)

There are two ways people might deal with this: They can learn to close themselves off from the chaos and let their unwanted abilities atrophy. Or they can learn to impose some measure of order on it, which enables them to control some of the spirits they are able to perceive.

Spirits spontaneously come into being in the chaos of the spirit realm. Usually, they dissipate back into chaos quickly, but some will gain enough will to find something that will anchor them. That is what sorcerers do: They bind spirits so that these beings will retain their shape and identity. In exchange, the spirit will do tasks for the sorcerer. Strong sorcerers can bind dozens or hundreds of spirits.

To create order in the chaos of the spirit realm, there are various methods: Rituals, mathematics, painted symbols, or music (that's what the Evening Sorceress uses).

Tl; dr: Sorcerers see (or hear, or smell) the spirit realm. They can impose order on the chaos there, and spirits will bind themselves to the sorcerers to profit from this order.