Amartya Sen has made significant contributions to economics, development studies and philosophy. His early work actually focused on collective choice, which was the the topic of his 1970 book "Collective Choice and Social Welfare" (re-printed with significant additions in 2017). In the 2017 Introduction, Sen outlines what social choice theory is and the broad array of questions it might be used to answer:

• "Challenges of group choice can be extensive and exacting, particularly because of the divergent interests and concerns of its members. Social thinkers have speculated, for a very long time, on how the concerns of the members of a society can be reflected in one way or another in the decisions taken in a responsive society (even if it is not fully democratic)... Social choice theory is a very broad discipline, covering a variety of distinct questions, and it may be useful to note a few of them as illustrations of its subject matter. When would majority rule yield unambiguous and consistent decisions? How can we judge how well a society as a whole is doing in light of the disparate interests of its different members? How can we accommodate rights and liberties of persons while giving due recognition to the preferences of all? How do we measure aggregate poverty in view of the varying predicaments and miseries of the diverse people who make up the society? How do we evaluate public goods such as the natural environment, or epidemiological security?" (p. 1).
  

The original text and the additional chapters follow a unique style - a chapter of context (the philosophical discussion) followed by a chapter of economics (the math). As a non-economist, this made the book easier to pick up. The book cannot be neatly summarized, as each set of chapters covers different questions (there are 15 pairs of chapters, and two concluding chapters). Many economists (as scholars in other fields do as well) seek a universal theory, however, Sen acknowledges a relatively high degree of subjectivity that seems necessary for society choice theories. He writes:

• "it is quite clear that an evaluation of the relative desirability of different systems will depend on the nature of the society. One way of interpreting the various 'impossibility' results is to say that there is no 'ideal' system of collective choice that works well in every society and for every configuration of individual preferences (as proposed by the use of the condition of 'unrestricted domain' employed in virtually all the impossibility theorems). Some choice procedures work very well for some types of choice and some sets of individuals preferences but not for others (see Chapters 5-7, 9 and 10), and naturally our evaluation of these procedures must depend on the type of society for which they may be considered. There is nothing outstandingly defeatist in this modest recognition." (p. 264)

Sen also reflects on the underlying assumptions and tools used within economics, and how these (often not reflected upon) has significant implications in influencing the field:

• "For well over a century welfare economics has been dominated by one particular approach: utilitarianism. It was initiated, in its modern form, by Jeremy Bentham (1789), and championed by such economists as Mill (1861), Sidgwick (1874), Edgeworth (1881), Marshall (1890) and Pigou (1920). Utilitarianism has been, in many ways, the 'official' theory of traditional welfare economics, and its tends to serve as the 'default programme' in mainstream welfare economic analysis: the theory that is implicitly summoned when no others are explicitly invoked. Utilitarianism combined what we have been calling 'consequentialism', 'welfarism' and 'sum-ranking'. It is a result-oriented (and in that sense, consequentialist) theory that concentrates only on utility consequences (which is the informational base identified by welfarism), and, in particular, focuses on the sum-total of utilities (which is the demand that sum-ranking makes)." (p. 341)

One of the new chapters links social choice theory to Sen's other work, such as his expounding on functioning and capabilities:

• "A person's achieved life can be seen as a combination of 'functionings' (i.e. doings and beings), and, taken together, can be the basis for assessing that person's quality of life. The functionings on which human flourishing depends include such elementary things as being alive, being well-nourished and in good health, moving about freely, and so on. It can also include more complex functionings, such as having self-respect and respect of others, and taking part in the life of the community... A person's 'capability' is represented by the set of combinations of functionings from which the person can choose any one combination. Thus, the 'capability set' stands for the actual freedom of choice a person has over the alternative lives that he or she can lead." (p. 357)

All page numbers from the 2017 Penguin publication.