  Though attractive, the proposal faces difficulties that have not so far been solved. One initial difficulty is to specify and justify the threshold that marks the personhood boundary. There are evidently different components of rational agency capacity, such as the ability to perceive features of one's environment, the ability to recognize and assess reasons for choice, the ability to deliberate and choose, the ability to form intentions and carry through what one has decided, and so on. To speak of rational agency capacity as one capacity presupposes some way of adding together an individual's component ability levels into one overall capacity. Then there is a line drawing problem: What justifies identifying the level of capacity that marks one as a person here rather than there? A perhaps more troublesome difficulty is that it is not at all obvious why the fact that people differ in their attainment of rational agency capacity above the threshold of personhood does not bring about difference in their fundamental moral status. Why doesn't the fact that you possess more of the very capacities for reasoning, insight, choice, and effective volition that qualified us as persons than I do render it the case that you are morally more considerable than I, have a higher fundamental moral worth? (see Carter 2011 and Waldron 2003). There is the further question how to determine what degree of moral worth or moral considerability attaches to beings who possess some rational agency capacities below the threshold level, or who possess capacities of some other sort such as bare sentience that should be regarded as moral considerability qualifications. And finally, Peter Vallentyne poses yet another worry (Vallentyne 2007). Suppose that we take rational agency capacity to be the basis of personhood. What then do we say about newborn human infants or normal young human children, who surely lack rational agency capacity at any plausible threshold level. If we say, the newborn human is a potential person, hence entitled to rights close to those of persons, the problem arises that in the future, when we have better genetic therapies available, and by applying expensive genetic treatment to a normal mouse, could endow it with rational agency capacity, the normal mouse looks likely to qualify as a potential person if the newborn or young human does.