In short, ideas built on first principles are based on reason.
When we ask, "How should I live?" We can take beliefs as our guide, or we can start from first principles.
Ethics is essentially the art and science of judgment. In the modern world, we largely think of ethics as morality, the difference between good and evil: "unethical" meaning "morally wrong." But in the ancient world, ethics was more than that. Ethics was our concern with how we behave not only in relation to other people, but also to ourselves.
Any work on ethics is an attempt to answer that question "How should I live?" If we divide life share by share, we must consider the value of one share over another alternative.