Constructivism in "practical philosophy" tries to thread a needle -- truth without realism. It seeks to establish that questions about what to do have right answers, but without appealing to a reality independent of us in which to ground them.
Part of why the constructivist argument is hard to make is our disposition toward a correspondence theory of truth. What makes a statement true under correspondence is simply that it accurately describes a reality independent of us or of our making the statement. Realism, in the sense of a reality independent of us, and truth seem joined at the hip.
But, in contrast to other forms of constructivism, constructivism with regard to right or reasonable action may have immediate intuitive appeal -- realism just isn't very attractive in moral or value theory. Intuitionism tried to ground moral claims in some sort of non-natural "real" properties, but intuitionism has never been especially convincing. In relying upon this fairly mysterious notion of non-natural properties, it paid a high price for moral realism.
Constructivism isn’t new to moral philosophy or value theory. The papers here reference Rawls’ constructivist theory of justice, and Rawls himself cited Kant as a source of moral constructivism. More recently Christine Korsgaard has championed her own brand of Kantian constructivism. Other writers here with strong, philosophically tested formulations of constructivism include Sharon Street, whose formulation is both presented and critiqued by several papers.
Many challenges to constructivism, including those here, pose worries about either infinite regress or circularity. In simplistic form, any given action A is justified by reference to previous justifications of actions that bear some relation to A. Those previous justifications likewise depend upon previous justifications, and so on. The chain can be kept from extending to infinity only by bending it back on itself into a circle.
Of course, constructivist theory is much more sophisticated, often appealing to more holistic, coherence based theories, presented and critiqued here, in which the justification of a particular judgement rests, at least partially, in its consistency and coherence with the body of judgements made by an agent, or by an agent and his community. Or justification is grounded in adherence to a process, much like Rawls’ theory.
The challenge undertaken here is not so much to defend constructivism against its alternatives as it is to make full sense of the constructivist claim. Arguments run to the technical side, both for and against -- the debate is here contained within the world of contemporary analytic philosophy. In the end, despite my own constructivist leaning, I found the challenges still daunting.