1. When evaluating an author's claims about the subjects above, one must recognize and understand the author's basic assumptions about how ethics is defined and what it is derived from (i.e. religion, nature, rationality, compassion, experience, etc.).
For example, if someone defines morality as human rules of conduct designed to promote human welfare and human goods, then the degree to which anything other than the interests of human beings is worthy of moral consideration is limited at the start.
2. You need to determine how the author is defining and using the terms above.
For example, Peter Singer believes that moral value comes from the ability to feel pain or pleasure. Tom Regan believes that inherent moral value comes from being a conscious individual with a life that has importance to itself regardless of its usefulness to others, i.e. being the experiencing subject of a life. Others believe that moral value comes from having a soul, or nature.Still others think that moral value is simply a social convention whatever society agrees upon.
3. Personhood and moral agency are distinct from moral value. It is not logically necessary to be a moral agent (an individual who can choose to act or not, and thus is responsible for his/her actions) in order to have moral value. It depends on the theory regarding how moral value is derived.
4. An individual can have both inherent (intrinsic) value and instrumental (extrinsic) value. For example, in E. B. White's book, Charlotte's Web, Wilbur the pig was valued in at least two ways. Charlotte, the spider, thinks that Wilbur has value in and of himself (inherent value). Farmer Zuckerman values Wilbur instrumentally, in terms of ham hocks, pork chops, and sausage. If nonrational beings, like fetuses and some animals, have only instrumental value, then they are only valuable to the extent that they are useful in gaining something else. They are valued because of the value of something they provide, not themselves per se.
5. Personhood is a status that grants individuals the highest level of moral value, according to many although not all (some recognize a higher level of Angels, others argue that all living creatures are equal).Persons have a right to life. They are inherently valuable, no matter their personality and character traits. Mary Anne Warren would grant that sentient animals and fetuses have some inherent value because they meet some of her criteria (listed below). However she would still allow them to be killed, if there is a compelling enough reason for doing so. In most cases, the interests of persons can override those of nonpersons, assuming that there is no reasonable way to respect the interests of both. For example, a pregnant woman's right to control her body would override any value of the fetus' life.
Mary Anne Warren uses a cognitive criteria of personhood. According to her, persons are beings who have the following traits:
Consciousness (of objects and events external and/or internal to the being), and the capacity to feel pain;
Reasoning (the developed capacity to solve new and relatively complex problems);
Self-motivated activity (activity which is relatively independent of either genetic or direct external control);
The capacity to communicate, by whatever means, messages of an indefinite variety of types, that is, not just with an indefinite number of possible contents, but on indefinitely many possible topics;
The presence of self-concepts, and self-awareness, either individual or racial, or both.
Warren believes numbers 1 and 2 are probably necessary conditions for personhood and probably 1-3 are sufficient. However, she does not insist that any of these are absolutely necessary. However, she claims that it is obvious that an individual that lacks all 5 is not a person.
Singer, P. Practical Ethics, 2nd Ed. Cambridge University Press (New York & Cambridge, U.K.: 1993).
Warren, M. A. "On the Moral and Legal Status of Abortion," The Monist, 57 (1973).