Original thought, self-awareness & intelligence on par with that of a human.
So those who are not "on par with that of a human" are ok to enslave? At what range are we speaking of? There are some animals that are smarter that some humans. Predators show adaptation and original thought. The hunt, devise hunting stratagies, learn from their mistakes, modify behavior to suit their prey. That is original thought.
How do you determine self-aware? It's not as simple a definition as you might think. How can you determine self-aware without the ability to communicate? What actions dictate self-awareness, since you have no way to communicate?
The best definition of self-aware that I could find is the Self-Awareness Theory:
Self-Awareness Theory states that when we focus our attention on ourselves, we evaluate and compare our current behavior to our internal standards and values. We become self-conscious as objective evaluators of ourselves. However self-awareness is not to be confused with self-consciousness. Various emotional states are intensified by self-awareness. However, some people may seek to increase their self-awareness through these outlets. People are more likely to align their behavior with their standards when made self-aware. People will be negatively affected if they don't live up to their personal standards. Various environmental cues and situations induce awareness of the self, such as mirrors, an audience, or being videotaped or recorded. These cues also increase accuracy of personal memory.
By that definition, certain dolphins, apes, and elephants fit that bill. Each are hightly intelligent, beyond that of some humans. Mental retardation, head trauma, psychological disassociative disorders all limit the intelligence of some humans. Are you comfortable enslaving them?
Once you try to define the rights of sentients, what sentience means, it makes is a slippery slope that can, and by some, will be used as a double edged sword.