  This paper is more an essay than a report. There is a gentle introduction to
some issues in modeling, followed by the use of steepest descent methods to
develop a model as contrasted to using such methods to solve one already in
hand, as in [1]. Three levels are discussed: fitting functions to model given
data, fitting an ODE to model given data, and more briefly, fitting a PDE to
model given data. Specific examples are discussed.
