You: Being part of a living creature does not imply life. The hair on one's head is not alive, and neither is the food in one's stomach.

Aaron: Hair is alive — it grows — and the food in one's stomach is not part of one's body any more than the air in one's lungs.

You: On the contrary, hair is not alive but merely a dead substance being pushed from the body. As for food, it becomes part of the body as it is digested. At what point would you say that the digested food springs to life?

Aaron: We are becoming lost in minutiae. Anyone can tell what is alive and what is not. Life is an intuitive concept.

You: I can see how your argument would be strengthened if this were true, and although we can generally agree on what is alive, if life cannot be defined then it cannot be used in your argument. For the purpose of a logical proof, life is no more "obvious" than morality.