I don't know if you agree meredith, but I don't think you've had an answer to your question yet - I especially like Thelands rejection of mechanistic behaviour without any reason or justification.
Maybe I can help with one.
I guess the point is that you need an element of randomness in the process to break what would otherwise be a truely mechanistic system.
Certainly 100 years ago it looked very much as you describe a mechanistic process, however since then we have two important advances.
Firstly quantum mechanics. The discovery that at it's very heart nature has a random essence.
The obvious question then is how can a process at a subatomic level affect a macroscopic object like a brain? If you think about the famous Schroedingers cat thought experiment this is exactly what is going on - admittedly this involves a particle detector but the principal is still there.
Maybe you don't like that one, well probably more interestingly there is non-linear dynamics - chaos theory to it's friends.
Here we discover "sensitivy to initial conditions" otherwise known as the butterfly effect.
Not to do with time travel, this was coined from the expression that the beat of a butterfly's wings in New York can cause a storm in London.
The point here is that some systems are so sensitive to initial conditions that the tiniest change in intitial conditions can be amplified into a major change.
Obviously the weather is one such system but an easier one to play with is the double pendulum.
Have a look at the applet here that shows this:
http://www.theanswerbank.co.uk/ChatterBank/Que stion538829.html
And especially the chart below, this is fractal, infinitely complex and hence the source of random motion