Q&A 6, First Answer

My question is: What variables in sentient beings are morally relevant?

Quite a lot, I think.  I am not necessarily qualified to provide a comprehensive list, certainly not without quite a lot of thought and debate with fellow ethics enthusiasts.  That said, I think certain variables are especially important, and (perhaps as a result of this) are also more readily apparent.  I can think of two in particular which stand out.

The first is the ability to feel pain, both physical and, in higher life forms, psychological.  The more acutely and complexly a creature is able to feel pain, the more ethically problematic hurting or killing that creature becomes.  Thus a shellfish, in spite of its biological status as an animal, has little to no moral value based on this variable.  It has the same moral value as a plant, in fact.  A mouse has far greater moral value, due to its ability to feel physical pain and psychological pain and panic.  A human has still greater value, as it can feel all the same things that the mouse can, but with far more complexity.

The second is the ability to reflect on one's own existence.  This one is trickier, as it is difficult to determine to exactly what extent various clearly sentient creatures are self-aware.  Regardless of its detectability, however, I think that self-awareness is a relevant component of moral value - if one can reflect on the value of one's own life, then one can place value upon it independent of what it may possess upon its own.