
I have a hunch that consistency of behavior is the key difference between "human"(e.g. economics) and "natural"(e.g. physics) systems. Once we can find a consistent pattern, we can build a theory of behavior because it satisfies the composition rule.

Remarkably, consistent pattern is a function of the Level of Abstraction (LoA). For example, in a Brownian movements each individual molecule behaves inconsistently when described at a LoA expressed in terms of its trajectory, which reflects interactions with other molecules. On the other hand, the entire system of molecules behaves consistently when described at a LoA expressed in terms of temperature. That is, within a certain range of temperatures the system behaves exactly the same when you heat it up for 1 degree, no matter what the starting temperature is. Therefore, we can describe the transformation of the entire system as a series of 1 degree transitions. (upd: the same approach can be applied to a Darwinian-style evolution).
In human systems, we achieve consistency by imposing external constraints on a large number of participants. For example, an assembly line imposes severe constraints on what operations an individual can perform (technology), what kind of parts s/he can use (design), and what the shapes of the parts are allowed (interchangeability). Once the constraints are in place, we can start predicting economic outcomes of the entire process.
Note that in natural systems the LoA that shows consistency (LoA-C) is discovered, while in human systems the LoA-C is created (upd: the difference between natural and artificial sciences).
Social institutions, such as legal system, roads, culture, etc., impose constraints on human behavior, making possible theoretical analysis, prediction, planning, and experimentation at different LoA-Cs. The latter represents the notion of freedom that depends on the state of institutions. For example, the freedom of having a self-driving car is a near reality in the US, but an almost impossibility in Russia because of the difference in the social dimension aptly described as "дураки и дороги."
upd: also consider forecasting vs backcasting