The philosopher John Searle famously argued that money is ontologically subjective, but epistemically objective. By “ontologically subjective”, he means that money depends for its existence on the subjective beliefs of individual people. A five-euro bill has no intrinsic worth that is, say, reducible to its material constitution; it is worth something only because people have a shared belief system about money. Yet, money is at the same time “epistemically objective”. Despite money’s subjective nature, if you estimate, say, my net worth, you can be right or wrong about this as a matter of objective fact.
Thus, while the study of money is typically left to economics, it is actually a quintessential psychological phenomenon. If anyone should be able to explain how such an odd thing as money can exist in the first place, it should therefore be psychologists. Such an explanation would have to rest on a theory that specifies how human beings can have shared beliefs, and that in turn would require a theory that explains how beliefs can have contents, that is, how it is possible that beliefs are about something. This feature is sometimes called “intentionality” – a term coined by the 19th century psychologist Franz Brentano.
Despite a century of progress in psychology and neuroscience, I don’t think anyone has a satisfactory account of intentionality. The concept has in fact often been ridiculed because it is hard to see how it could have a basis in physical reality. The laws of physics do not contain anything that resembles it: physical theory explains how and why the moon orbits the earth, but there is no sense in which physical states of the moon are “about” the earth. Physical relations between objects may be governed by laws that support their causal interaction, but that’s about it. So, if you believe that ultimately only physical entities exist, then subjective mental states will turn out not to exist except as purely physical states that are fully described by specifying their causal interactions. This makes Searle’s idea that money (or anything else) exists as an ontologically subjective entity moot: money isn’t ontologically subjective, because the subjective beliefs that should support it don’t really exist at all. In the end, there’s only particles, fields, energy – the rest is an illusion.
I have always found this position deeply unsatisfactory. The fact that we can somehow have shared beliefs about things like money, religion, and philosophy is one of the most distinctive features of our species and it is what makes human society possible. Most of these mental states are in addition highly robust. Every single language that ever existed allows us to express the fact that people can “know”, “want”, and “think” certain things, and we have no trouble whatsoever in translating the contents of mental states across languages, however old they are: while it is difficult to even understand the theories of physics that existed before Galileo, we can understand the mental states of characters in Gilgamesh or the Bible without the slightest effort. It is our scientific duty to explain why our mental states have contents, not to explain them away with a vague reference to the ultimate physical description of reality. The subjective ontology of money is therefore among the most important phenomena awaiting psychological explanation.