What is sentience?  Or more precisely, what is the aspect of people that I think separates them from mere animals?  Now, it’s possible that you believe humans aren’t different than animals in any significant biological way – that all of the things we define as “sentience” is really just emergent behavior from the bio-machine – but I think it’s still worthwhile to define the encapsulation of a “sentient person” and compare that definition to a normal biological organism.

I think that sentience includes self-awareness.  This is extremely hard to measure objectively, but as will be apparent as a common theme, I think there can be objective qualities to the universe (or stuff in the universe) that we can’t demonstrably prove, but we can subjectively suppose and analyze, and that activity has meaning. I think sentient beings believe themselves sentient, and that also that they think (or think they think) in ways qualitatively different than those required for survival (e.g. selected for directly).

I also think that sentience implies the capacity of choice.  That is, a sentient being can make a decision given two possible courses of action according to internal reasoning (rational thought).  Other animals will demonstrate behavior that looks like choice, but can actually be attributed to conditioning.  I’m not saying sentient beings always make “choices” (sometimes they are also conditioned) but I do think animals never do.

All this being said, I think sentience is probably more like a spectrum then the binary on/off I represent it as, here.  Just as we can use many pieces of data observed about a person to try to tell whether they are essentially sentient, we can also apply those pieces to moving them around in the spectrum of sentience.