INTRODUCTION to CONTEMPORARY THEORIES of NATURAL LAW
Any attempt to summarize and compare theories of natural law from the
 early twentieth century to the present day inevitably becomes entangled
 in the debates between the theories themselves.  Nevertheless, it is 
illuminating to see contemporary theories of natural law as falling into
 two general categories:
- Neo-Thomism
- The Grisez, Finnis and Boyle Theory
Neo-Thomism is the vigorous and complex revival of the thought of the
 thirteenth century Dominican theologian St. Thomas Aquinas, who is 
perhaps the paradigmatic natural law thinker.  The most important source
 of the neo-Thomist revival was Pope Leo XIII’s 1879 encyclical letter Aeterni patris,
 in which Leo called for the rejuvenation of Christian philosophy and 
proposed St. Thomas Aquinas as its exemplar.  In the decades following 
Leo’s call, neo-Thomism coalesced into a number of contrasting schools 
that emphasized different aspects of Aquinas teaching, or focused on 
meeting the challenges of different modern philosophers such as Kant or 
Husserl.  Key early figures in neo-Thomism are Reginald 
Garrigou-Lagrange, Charles De Koninck, Joseph Maréchal, Etienne Gilson, 
and Jacques Maritain.  All of these figures were Catholics and most were
 clerics; concern for the natural law was just a part of their concern 
for elaborating a comprehensive philosophy and theology. 
Garrigou-Lagrange presented a Thomism that was indebted to the 
Scholastic commentary tradition; De Koninck emphasized Aquinas’ 
Aristotelianism; Maréchal reworked Aquinas to engage Kant and Descartes;
 Gilson and Maritain stressed, in different ways, the priority and 
distinctiveness of Thomistic metaphysics.
Maritain’s work on the natural law had the greatest influence on 
American thought.  He taught in the United States during and after the 
Second World War and was involved in politics at high levels (in 
particular the drafting of the UN Declaration of Human Rights).  In some
 respects Maritain’s political writings anticipated the work of Harvard 
philosopher John Rawls.  More recent figures who have developed 
neo-Thomism with a more exclusive focus upon natural law include: 
Russell Hittinger, J. Budzisewski, Ralph McInerny, Henry Veatch, and 
Martin Rhonheimer.  With the exception of Veatch, these neo-Thomists 
also self-consciously work within the Catholic tradition.  
Another, although smaller and less well-defined, strand of 
neo-Thomism originated in Anglophone philosophy with the work of Peter 
Geach, Anthony Kenny, G. E. M. Anscombe, Herbert McCabe, Alan Donagan, 
Mark C. Murphy, Eleonore Stump, Anthony Lisska, and Alasdair MacIntyre. 
 Although these thinkers have been influenced by the Catholic mainstream
 of neo-Thomism, many of their preoccupations and concerns are native to
 the analytic philosophical tradition that arose with Frege, Russell, 
and Wittgenstein.  Many of the ‘analytic’ neo-Thomists, in particular 
MacIntyre, have made important contributions to contemporary moral 
theory.  The relationship of these contributions to the natural law 
tradition is contested, however, because they are often framed in terms 
of “virtue theory,” and the compatibility of an ethic of virtue with a 
morality of law is a debated issue.
The second category of contemporary theories is the version of 
natural law theory developed in collaboration among Germain Grisez, John
 Finnis, and Joseph Boyle.  It can be seen as part of the broader 
neo-Thomist revival, but also as friendly opposition to it.  The Grisez,
 Finnis, and Boyle theory is inspired by Aquinas, but is ultimately a 
novel philosophical enterprise that rejects central tenets of Aquinas’s 
thought, and is noteworthy for its systematic character and engagement 
with practical moral questions.  The theory emphasizes the priority of 
practical knowledge, as opposed to speculative knowledge of nature, in 
moral theory.  Other collaborators in the Grisez, Finnis, and Boyle 
project include Robert P. George, Patrick Lee, and Christopher 
Tollefsen.