Basically what I asked Eliezer: What sense of the word "sentient" is he using, such that babies plausibly don't qualify? My de facto read of the term and a little digging around Google show two basic senses:

-Possessing sensory experiences (I'm pretty sure insects and even worms do that)
-SF/F writer's term for "assume this fictional entity is a person" (akin to "sapient"; it's a binary personhood marker, or a secularized soul -- it tells the reader to react accordingly to this character's experiences and behavior)

The latter, applied to the real world, sounds rather more like "soul" than anything coherent and obvious. The former, denied in babies, sounds bizarre and obviously untrue. So...I'm missing something, and I'd like to know what it is.