We don't know what sentience IS exactly, as we have a hard time defining it. We assume other people are sentient because of the ways they act. We make a judgment based on behavior, not some internal state we can measure.
And if it walks like a duck, quacks like a duck... since we don't exactly know what the duck is in this case: maybe we should be asking these questions of 'duckhood' sooner rather than later.
So if it looks like a human, talks like a human... maybe we consider that question... and the moral consequences of owning such a thing-like-a-human sooner rather than later.
