We might worry that there is a problem of circularity in claiming that there is a moral interest driving the specification of ‘person’ as range property. Surely such a moral interest exists only if all human beings within the range really are morally equal. But this is exactly what an account that grounds personhood on graduated characteristics seems to deny.
In a discussion about the basis of human equality, Ian Carter considers this challenge as it applies to the range property solution.  The problem, Carter explains, is that of showing why we should concentrate on the range property itself, rather than on the more basic scalar properties. We require, he says, some ‘independent moral reason for focusing on the range property’, a reason that, to avoid circularity, must be independent from any prior commitment to equality.14
Carter suggests that a moral reason of this nature can be found in the necessity for what he terms evaluative abstinence, meaning, ‘a refusal to evaluate persons’ varying capacities’, relative to their classification as persons.15 The need for evaluative abstinence stems from the conditions of showing respect for human dignity. Showing respect for persons, he argues, involves adopting a certain perspective, one which avoids looking at their variable qualities and capacities when assigning them basic moral respect. He writes:
Respect, on this alternative interpretation, is a substantive moral attitude that involves abstaining from looking behind the exteriors people present to us as moral agents. More precisely, while we may see behind these exteriors (for to do so is often unavoidable), if and when we do perceive people’s varying agential capacities we refuse to let such perceptions count as among the reasons motivating our treatment of those people.16
14 ibid 550.
On Carter’s view, respect for human dignity requires maintaining a kind of blindness towards individuals’ varying capacities. It requires, in his words, treating them as ‘opaque’ in many respects. The outward dignity of a person is, he argues, compromised by a certain kind of appraisal by others. Humans lose that dignity when they are ‘inappropriately exposed, which is when they are evaluated in respect of features which ought not to be evaluated in the given context. The realization of outward dignity requires, he argues, ‘a degree of concealment’ in the way of inaccessibility to features that pertain to particular agential capacities.18 Carter terms this external perspective ‘opacity respect’. He claims that opacity respect is the appropriate attitude to adopt above a certain absolute minimum possession of the relevant capacities. Opacity, we might say, is instrumentally indispensible to sustaining the sort of relations that Carter thinks of as inherently morally valuable: relations in which the community of persons treat each other with dignity.
Carter makes clear that the importance of evaluative abstinence does not render all engagements in internal evaluations of others inappropriate. Some such evaluations are apposite when people occupy certain roles. ‘I can’, he explains, ‘assess a person’s intellectual capacities as insofar as I relate to her as a professor’.w Likewise, it is not inappropriate when medics and psychologists assess our physical and mental capacities. Human dignity does not require total blindness to each other’s differences in all contexts. But it requires the refusal to focus on many of them in many contexts, and especially insofar as we relate to each other as morally considerable persons.
Thus, respect for persons and their outward dignity grounds a moral need to treat personhood as a range property. Carter’s analysis explains why, above an absolute minimum, ‘opacity respect’ necessitates a range property analysis, under which interpersonal variations in person-relevant capacities do not affect the equal moral status of individuals within the range. These reasons to treat personhood as a binary category above a minimum threshold are not merely instrumental, but morally essential for maintaining the kind of relations between persons that we deem valuable. To be a person is to be the sort of being whose specific cognitive or emotional capacities are irrelevant for the basic appraisal of her standing as a creature worthy of the strongest moral protection. This same idea is, of course, embedded in the philosophy of those who support conception as the qualification threshold for personhood. Proponents of the conception threshold also treat personhood as a binary property within a specified range. Where they differ from others is in how they specify the boundaries of that range.