Philosophy and economics studies topics such as public economics, behavioural economics, rationality, justice, history of economic thought, rational choice, the appraisal of economic outcomes, institutions and processes, the status of highly idealized economic models, the ontology of economic phenomena and the possibilities of acquiring knowledge of them. It is useful to divide philosophy of economics in this way into three subject matters which can be regarded respectively as branches of action theory, ethics (or normative social and political philosophy), and philosophy of science. Economic theories of rationality, welfare, and social choice defend substantive philosophical theses often informed by relevant philosophical literature and of evident interest to those interested in action theory, philosophical psychology, and social and political philosophy. Economics is of special interest to those interested in epistemology and philosophy of science both because of its detailed peculiarities and because it has many of the overt features of the natural sciences, while its object consists of social phenomena. Scope Definition and ontology of economics The question usually addressed in any subfield of philosophy (the philosophy of X) is "what is X?" A philosophical approach to the question "what is economics?" is less likely to produce an answer than it is to produce a survey of the definitional and territorial difficulties and controversies. Similar considerations apply as a prologue to further discussion of methodology in a subject. Definitions of economics have varied over time from the modern origins of the subject, reflecting programmatic concerns and distinctions of expositors. Ontological questions continue with further "what is..." questions addressed at fundamental economic phenomena, such as "what is (economic) value?" or "what is a market?". While it is possible to respond to such questions with real verbal definitions, the philosophical value of posing such questions actually aims at shifting entire perspectives as to the nature of the foundations of economics. In the rare cases that attempts at ontological shifts gain wide acceptance, their ripple effects can spread throughout the entire field of economics. Methodology and epistemology of economics An epistemology deals with how we know things. In the philosophy of economics this means asking questions such as: what kind of a "truth claim" is made by economic theories – for example, are we claiming that the theories relate to reality or perceptions? How can or should we prove economic theories – for example, must every economic theory be empirically verifiable? How exact are economic