Brains are of different sizes, so can be any size. What happens when the size is 0? The data flow through any active thing is: nature -> sensors -> brain -> motors -> nature Remove Brain and you have: nature -> sensors -> motors -> nature The functions here have a functional complexity of 1. To get above one in complexity you must add a brain. (functional complexity = sensors/motors.) Notice that a brain has flow from motors to sensors. Sensors remove units of measurment. Motors add units of measurement. So a brain reads numbers, adds units of measurment if needed, does a computation, then becomes a sensor and removes units of measurement again. Replace brain with neuron or with node and all this above remains true. Valentino Braitenberg Wrote a book "Vehicles", MIT Press 1994 "Experiments in Synthetic Psychology" is what he called its contents. To bad he did not know how to make a brain. I made some brain code for all the vehicles that had proper description. I sent this to a reviewer of the book. Got no response of any kind. Perhaps it is good that he did not know how to build a brain. It has not helped me.