b) What is the philosophical explanation to the necessity of multiple theories to describe and have a better understanding of our world?
There is no absolute necessity of multiple theories. The necessity is, as always, a consequence of something else being the case. The something else is the inevitable limitation of our own brain, therefore of our mental processes, and of our representations.
Intelligence works by processing bits and pieces of information which have a maximum size. Yet, we are trying to represent as much as possible of the whole world, starting with parts or aspects of the world that most interest us. These are far larger in terms of the quantity of information required than the bits and pieces our intelligence is able to manage.
A simple example of that is the method we use to add two big numbers. First, we represent each big number as an ordered set of smaller, one-figure numbers, we call digits, so that we can divide the addition of two big numbers into several smaller additions of just two digits which our brain can manage. This is basically what we do whenever we think in a rational way: mathematics, philosophy, politics, etc. Any time we use a formal language, words or symbols, we have to rely on this principle.
The overall result is that our representation of the world is structured in levels of abstraction, from the brute perception of our immediate environment, to fluffy notions of democracy, necessity, happiness, things, and indeed economics.
It should be possible in principle (in the "abstract") to represent the whole of reality completely with such an abstract model. In practice of course, there are serious difficulties. The whole progress of civilisation goes in parallel with a general inflation of our abstract representation of the world, and this while our natural capacity of perception of our environment remains broadly identical to what it has been for the last perhaps 400,000 years.
This explains why we have to divide our expertise among many different experts. We have now entire professions dedicated to working full time on our abstract models of the world, each profession working on only one very limited aspect of it, and at one or two level of abstraction. Physics at the fundamental level, mathematics and philosophy at the most abstract level, and the various sciences, including engineering and economics, in between.
Each discipline deals inevitably with at least two levels of abstraction. In the equation E = mc2 there are already several levels of abstraction. Physicists may disagree, I'm sure, but, yes, several levels. Representation is inevitably representation of something in terms of something else, and what is represented is more fundamental, less abstract, and there are therefore always at least two levels of abstraction.
Our models are often inaccurate, inexact, incorrect, or somewhat confused, but we work constantly to improve, simplify and clarify them. In principle, we could do models involving only two levels of abstraction but our models are meant to be pragmatic and practical rather that metaphysically correct.
a) What is the necessity in many theories in the human sciences to understand our world, with regards to economics?
Economics is far removed from physics and therefore a highly abstract science. This means first that our models are all probably flawed: inaccurate, inexact, incorrect, or somewhat confused. They are also complex, which requires many experts and many specialities to cut up the hard work into manageable pieces.
Again, this is not necessary in principle. This is merely a consequence of our human nature and we cannot change it. And any cognitive beings, either here on Earth or somewhere in distant galaxies are most likely subjected to this limitation.
Our own brain is made up of, divided into, very small pieces we call neurons, all interconnected and organised to effectively divide the load of the necessary processing of perception data and management of memorised data into workable operations.
Economics involves very many aspects of the real world, even if essentially limited to this one planet. Broadly, all the other disciplines outside economics have or could have some inputs into it: all other empirical sciences, mathematics, logic, sociology, technology, medical research, politics, human psychology, epidemiology, meteorology, now climatology, drug trafficking, armed conflicts, and even philosophy. Many people, for example, are changing the way they consume as a result of what climatology now tells us. This already affects the real economy and may have to be taken into account at some point in our theoretical models of it.
This works also in the other direction. The economy affects our perception of the world and therefore economics potentially affects our more abstract models, such as our models of political organisation, the moral models we have of ourselves, even the sense we have of our place in the universe, beginning with our perception of the effect we have as a species on our immediate environment, the Earth and other animal species. Homo Economicus is driving our politics at all levels.
One crucial point is that we need to focus on the aspects of reality that are the most relevant. We have to prioritise, again because we are unable to do everything at the same time. So, the important question is that of the factors that most affect the economy. My feeling is that we tend to focus too much on factors that more visibly effect the economy because they are short-term factors. This is a bias, again a consequence of our own data processing limitations.
