Systems thinking is all about wholes as opposed to parts. Put another way, systems thinking is profoundly ecological thinking.
According to the "systems view", the essential, important properties of a system are properties of the whole, which none of the parts have individually. The essential, important properties of the whole arise from the interactions and relationships between the parts.
This is a departure from normal convention of course: the belief that the behaviour of a complex system can be understood entirely from the properties of its parts is central to the "Cartesian Newtonian" worldview that has defined much of Western thinking for many hundreds of years, and is prevalent in much education practise, management consultancy and academic research.
Indeed, the great shock of 20th century science specifically, and of other disciplines more generally, has been that complex systems like the weather, DNA, financial markets or social systems cannot be understood by analysis. This realisation has been reached independently by a number of different people via a number of different routes: quantum physics, complexity theory, cybernetics, philosophy and biology have all in different ways undergone a "systems" revolution in the latter half of this century.
What is a system?
There is a danger of course that the term "system" becomes vacuous through repeated and ill-defined use. What I mean by a "system" is a pattern, or network of relationships, between things. This is still a very loose definition, encompassing living organisms, commercial groupings and weather systems, to name but a few.
The truth is, everything is a system, in one way or another. Everything is a network of relationships between things. The challenge, as I see it - to borrow Bateson's phrase (and I still don't fully understand the full implications of this) is, in all things, to find the "pattern which connects". Once you have that, you have the beginnings of a systems understanding of the world.
Three tenets of systems thinking
Systems thinking and organisations