No one disputes the necessity of models. Why do you think I did?<p><i>I didn&#x27;t say that. I said that we can&#x27;t predict with certainty...</i><p>Actually you said &quot;I cannot say that my predictions are better than random&quot;. Either your predictions convey information (i.e., posterior(world state|prediction) != posterior(world state)) or they doesn&#x27;t. If they don&#x27;t, they are useless for prediction since E[utility|choice] cannot vary.<p>If they do, you can make the basket of predictions I asked for and behave better than random chance.<p><i>Really? If your measurement time (of initial conditions) significantly exceeds the time the predicted trajectory is within usable error, then yes -- nonexistence of predictions does follow.</i><p>The nonexistence of stable predictions of x(t) does indeed follow. Again, there are a huge number of quantitative predictions which can be made. For example, many systems with positive liapunov exponent allow claims like &quot;a particle&#x27;s long term distribution will behave like a sample from the distribution f(x) dx&quot; or &quot;a particle with energy &lt; E_0 will travel a distance L only with probability &lt; A exp(-BL).&quot;