What's a Soul?

I had a conversation with a co-worker the other day about my dogs and the difficulty I was having with them and their obedience. Due to a bout of frustration, I hit Kai on the nose and felt really really bad about it. My coworker said, "Well that's ok because Dogs don't have a soul." Without thinking I responded with, "Of course they do, every living thing has a soul/spirit." She defined a soul as your "will" or ability to make a "choice" which is interesting but I think Dogs do choose and have a will. So Jake and I talked about it at the park today and he thought humans and animals have a spirit because they have a brain but plants and other organisms have "the essence of life or energy." That makes sense because plants that are talked to seem to flourish and I don't think it is because of the actual conversation but the energy you are investing into the plant. With that understanding I then asked him, "If you're not dead then what are you?" and he replied, "Alive." Which I then asked, "What is it that makes something alive and you can't just say because it's not dead." Jake's thought was anything that grows. So take a cellular process, I suppose it grows but I'm just still not sure on this. What makes something alive or is the essence of life and what has a soul/spirit?