[PaulDavisThe1st]

Possibilities I'm aware of from the philosophy of consciousness field:

1. a unique property of a specific type of biological tissue, not substrate independent

2. pan-consciousness: everything is conscious, but degrees of it vary, it's as much a part of the universe as gravity

3. a soul - some of sort of non-physical entity that confers consciousness but is specific to an individual

4. external consciousness: brains as receivers for consciousness, not the generator

5. specific only to humans and not substrate independent (i.e. rooted in physiological structures but not replicable in non-human form)

There may be more.

[dtj1123]

Apart from 2, I've either come across convincing arguments against these or would consider them essentially appeals to magic.