Just as life tends to exist at the boundary between order and chaos, so free will may thrive at the boundary between necessity and chance, between excess rigidity and excess fluidity.

This is too vague as stated. Let us try to firm it up by some analogies to thermodynamics.

The fundamental equation in thermodynamics is

H = F + TS

where H is the enthalpy or total energy, F is the free energy (available for use), T is the absolute temperature, and S is the entropy or tendency to disorder. TS then represents the unavailable or degraded energy.

As a mental picture, we can think of H as representing the electromagnetic forces that hold molecules and crystals together and TS as the random thermal motions which tend to disrupt these structures. Then F is what is left over of order after the disruptive forces have had their play. If TS is greater than H, the substance will be a gas, totally disordered. If TS is much less than H, the substance is a crystalline solid, with considerable amount of order. If H and TS are almost equal, but with a slight preponderance of H over TS (i.e. at the boundary between order and disorder), F will be small but positive. Could F under these conditions represent the (limited but nevertheless real) scope for free will as navigating between necessity and chance?

Could we maximize free will F by making TS much smaller than H? The equation would seem to indicate this; but probably the quality of the larger F would change – it would become more “H-like”, i.e. rigid. It seems that TS needs to be large enough to loosen or “plasticize” the rigid order, but not so large as to shatter or vaporize it.

Another way of looking at it: Let H represent the total behaviour; then F is the part that I can manipulate, TS is the part that I cannot manipulate.

We can act according to rigid rules, or we can act capriciously and unpredictably; but the best (most free) choice is the golden mean in between.

As to the mechanism of action of voluntary choice, the most likely candidate is quantum-mechanical tunnelling which is ruled by probabilistic laws, so that “I” (whatever that entity is) can act by slightly weighting the dice.