Tuesday, March 11, 2003

Simulation and Its Discontents, Part III

At the turn of the century, a book came out that gathered quite a bit of attention. It said that it could explain the vast complicated mechanisms of life, and startlingly, it could do so with only a single, simple principle. It sold quite well, and was received well in the popular mind (although less so by the scientific community). Its most striking feature was the beautiful pictures it included, showing what complex and sophisticated forms could evolve given only this one, basic principle.

I'm speaking, <irony>of course,</irony>, of Stephane Leduc's The Mechanism of Life, published in 1911. The principle he claimed was basic to all life? Osmosis. And the pictures really did look like the metabolic states of a living cell.

The point is, of course, that he was wrong. (And if you thought I was talking about another book, well, the jury's still out but the parallels are eerie.) Just because you can construct a mechanism that produces results that, superficially, look like the large-scale behavior of the system you are trying to model, doesn't mean you have a true model of a system.

In fact, we must recall statistician George Box's admonition: "All models are wrong, some are useful." Does the model have similar fidelity of data as the real system? Similar sensitivity to changes, to initial conditions? Does it capture scaling effects? Too many simulations claim congruence with a target system without demonstrating in a fundamental way that there is in fact underlying similarity in behavior.

(Note: I am indebted to Evelyn Fox Keller's wonderful book Making Sense of Life for the story of Stephane Leduc.)