Is the middle game one big fight? That's a reasonable view, in that central fighting can affect just about any part of the board except fully-enclosed corners.
However, consider some basic middle game phenomena:
- several weak groups, some more important to save than others;
- several invasions possible in different frameworks.
One can try to sort out one's options by some modular thinking about this. Break the board down mentally into separate fighting areas first, and try to understand the relative importance of plays in relation to each (e.g. adding one stone to improve the eye shape of a weak group, playing an invasion in a framework before it's too late). Then try to integrate these separate pictures of fights going on concurrently. The hope is to extract a sensible direction of play.
The 'concurrent' way of looking at this says: White has a good invasion at a to live, but also a weak stone in the middle of the side. is played in the boundary region between the two fights White would like to participate in. Whether or not it is good, it is a hard-working idea.
One can unify weak groups and invasions conceptually: think of invasions as virtual groups (weak groups you haven't yet made - the name is due to Matthew Macfadyen). Then the concurrent fight picture is of a board full of (virtual and actual) groups, all in the course of living and dying.