I recently wrote about Steven D. Smith’s arguments in The Disenchantment of Secular Discourse, and more specifically, about how he demonstrates the vacuity of many of the Western world’s central political buzzwords, such as equality and freedom. In the course of his critique of Martha Nussbaum, he mentions another one that I thought deserved its own post: “dignity”, or as it is sometimes also called, “worth”.
Smith asks: “What is ‘human dignity,’ exactly, and how is it that human beings come to have it?”  He clarifies that the terms can have an obvious and pedestrian meaning, one that defines a quality people sometimes have and sometimes do not. For example: “A grave, calm, soft-spoken gentleman may be described as having ‘dignity,’ but it would seem odd or even laughable to use the word with respect to a rambunctious child, or a vulgar, clownish teenager.”  He concludes this is obviously not what human rights defenders mean by the term. This is clearly correct, because the entire point of the human rights version of “dignity” is to be a quality that everyone possesses such that they deserve certain respectful treatment.
Smith notes in various ways that this concept in ethical thinking has its roots in more religious and metaphysical thinking, where it points to something like the image of God in human beings. [178-179] At the same time, such justifications are supposed to be impermissible in political discourse in contemporary secular contexts. This raises the obvious question, though: can the concept of dignity do the work human rights supporters want it to do absent any metaphysical or religious foundation? In response to this question, Smith notes many naturalistic philosophers recognize the tension and with clear eyes have responded in the negative. 
Insofar as political discourse today disallows appealing to principles that a secular naturalist would not assent to, and insofar as secular naturalism defines the natural world as metaphysically stripped of value and purpose (i.e., of Aristotelian final causes with their divine causal ground), it seems impossible to explain how “dignity” (that is, intrinsic moral worth or value, or, because they are the same thing, intrinsic and objective moral goodness) could be granted as an objective fact that supports human rights claims. On the contrary, it seems to be precisely the kind of thing that cannot be appealed to by the rules of secularism.
Grimly, what can be seen through these secular/mechanistic eyes are some similarities and some differences between human beings, e.g.., differences in bodily strength, intelligence, history, sex, etc. What cannot be seen in the quantifiable world is a categorical reason to treat them them equally or the same. Yet, it is a foundational custom of Western political thought that all people are equal in dignity or worth, and indeed it is customary not to just uphold the concept, but to uphold certain practices as correlates of the concept. And thus “dignity” becomes exactly what Smith classifies it as: a cipher used to smuggle in practical conclusions based on metaphysical positions to which people are not permitted to expressly appeal.
I’ve written in the past on how the concept of “equality” is similarly afflicted with ambiguity. (And to give credit where it’s due: I learned of Peter Westen’s article through Smith’s work.) And yet, as I noted in that older post, for Christians, there is a way out of the ambiguity. Because we hold that there is an objective created order made by a real Creator who objectively calls that order “very good”, valuable, and worthy of respect.