Can a person make true prophecies without using magic?
Sybil Trelawney, the seer introduced in the books doesn't seem to do spells in all the seven books. She even hides her empty sherry bottles in the room of requirement instead of using the evanesco spell.
Why would an adult wizard who is also a teacher risk getting caught with empty bottles instead of just vanishing or transfiguring the evidence of her being a lone drunk?
The two times she made her prophecies it seems she wasn't even aware of it, rather than being a conscious act, it seemed she was possessed.
Is it possible that she is a squib, and some other entity was communicating through her? Can a seer be a squib at all or a muggle or does one have to use magic to make true prophecies?