# The Notion of Computation

## Computation as a Framework

In earlier parts of this book we saw many examples of the kinds of behavior that can be produced by cellular automata and other systems with simple underlying rules. And in this chapter and the next my goal is to develop a general framework for thinking about such behavior.

Experience from traditional science might suggest that standard mathematical analysis should provide the appropriate basis for any such framework. But as we saw in the previous chapter, such analysis tends to be useful only when the overall behavior one is studying is fairly simple.

So what can one do when the behavior is more complex?

If traditional science was our only guide, then at this point we would probably be quite stuck. But my purpose in this book is precisely to develop a new kind of science that allows progress to be made in such cases. And in many respects the single most important idea that underlies this new science is the notion of computation.

Throughout this book I have referred to systems such as cellular automata as simple computer programs. So now the point is actually to think of these systems in terms of the computations they can perform.

In a typical case, the initial conditions for a system like a cellular automaton can be viewed as corresponding to the input to a computation, while the state of the system after some number of steps corresponds to the output. And the key idea is then to think in purely