What is a reductive theory?

Question

What makes a theory reductive? Simple explanations please.

I may well have posted this in the wrong section, wasn't sure if it belonged in philosophy.

Answer 1

Philosophy might be the best section for this.

The word "theory" is being used in its technical sense; as an explanatory framework.  This is completely different from the common use of the word to mean a hypothesis.

A reductive theory is one that explains the properties of a system in terms of the properties of its simpler components. Thus, we know in principle that chemistry can be reductively explained in terms of the physics of electrons moving around atomic nuclei.

This is as opposed to theories that recognise the emergence of new properties as the system becomes more complex.

In some areas, reduction may in principle be possible but in practice be unworkable.  If chemists had to go back to the behaviour of electrons all the time, they would never get anywhere.  

In other areas, reduction may completely miss the point.  At this stage, the system is said to have "emergent properties".  These are not mysterious violations of the laws governing the behaviour of the simpler components, but arise from the complexity of the overall structure. For example, the theory of prey-predator interactions in ecology. This cannot be reduced to the behaviour of individual organisms, however well we understand them.

Answer 2

This belongs in the Philosophy section...

Reductive theory is basically everything in the world can be reduces to one or a few basic things.

Difference combinations and amounts make difference objects.