There has in the past been considerable confusion about why this might be the case. But the key to understanding what is going on is simply to realize that one has to think not only about the systems one is studying, but also about the types of experiments and observations that one uses in the process of studying them.

The crucial point then turns out to be that practical experiments almost inevitably end up involving only initial conditions that are fairly simple for us to describe and construct. And with these types of initial conditions, systems like the one on the previous page always tend to exhibit increasing randomness.

But what exactly is it that determines the types of initial conditions that one can use in an experiment? It seems reasonable to suppose that in any meaningful experiment the process of setting up the experiment should somehow be simpler than the process that the experiment is intended to observe.

But how can one compare such processes? The answer that I will develop in considerable detail later in this book is to view all such processes as computations. The conclusion is then that the computation involved in setting up an experiment should be simpler than the computation involved in the evolution of the system that is to be studied by the experiment.

It is clear that by starting with a simple state and then tracing backwards through the actual evolution of a reversible system one can find initial conditions that will lead to decreasing randomness. But if one looks for example at the pictures on the last couple of pages [442, 443] the complexity of the behavior seems to preclude any less arduous way of finding such initial conditions. And indeed I will argue in Chapter 12 that the Principle of Computational Equivalence suggests that in general no such reduced procedure should exist.

The consequence of this is that no reasonable experiment can ever involve setting up the kind of initial conditions that will lead to decreases in randomness, and that therefore all practical experiments will tend to show only increases in randomness.

It is this basic argument that I believe explains the observed validity of what in physics is known as the Second Law of Thermodynamics. The law was first formulated more than a century