Reductionism is a process of reasoning used to describe things by breaking them down into their constituent components, analyzing the properties of these components in isolation and then recombining them in order to get a description of the whole system as a set of its individual parts, their properties and the linear relations between them. This is a process of reasoning that comes very natural to us, we can take anything from a planet, to a bird, to a car or business organization and use this method of reasoning in order to understand how it works and then from this we are able to change its functioning by changing the parts or how they interact, this process of reasoning is also called analysis and it is central our engineering of technology.
There is of course another method for describing things called synthesis, instead of breaking things down, with synthesis we put them together, that is to say we look at the system within the context of its environment and its relations to other systems in that environment. From this perspective, a bird is the way it is because of its place within the ecosystem it is a part of. The bird has wings because of the makeup of the atmosphere, water resistant feathers because of the nature of water, and a beak because of the structure to the other creatures or plants it preys upon, it is the product of its relations and functioning within the environment it is a part of. Syntheses recognizes how new levels of organization emerge at different levels because of the interaction between the system and other systems within it environment.
These are two very different ways of seeing the world and unfortunately, they don’t always sit too comfortably with each other. The aim of the game within the reductionist paradigm is to break things down in order to itemize their parts; the whole is thought to be nothing more than the sum of its parts. Thus once we have this set of parts and know how to put them together we can then of course change the parts in order to alter the functioning of the entire system, we can alter the parts in order to optimize the system to our advantage, and this is what we call engineering. Once we understood the workings of plants within the biosphere we could create farms, once we understand atoms we could engineer new molecules and now that we understand DNA it is thought that we can do bioengineering.