The "THIS" Riddle

I can be this, and you can be
this. And yes, we can be this. He
can't be this, and she can't be
this. And no, they can't be this.
Dogs can't be this, but cats can.
And a kitten can't be this, but a
puppy can. Givers can't be this,
but beggars can. And humility
can't be this, but greed can. Not
even peace can be this, but fear
can. What is "THIS?"