ABSTRACT
To discover the hidden structure in what we observe is a source of great enjoyment and a worthy goal for scientists to achieve. This structure is often revealed by successively employing models of increasing elegance and generality. The physical attributes of any set of objects will never occur at exactly the same value. Similarly, the chemical fluctuations in time and space around each cell, the cacophony in the living world, ensure that mental representations for the same and different objects will not be identical. Explicit models for these fluctuations might be based on known molecular-cellular processes and principles. In the absence of this knowledge, it is often useful to employ models that can be justified on the basis of experience in model fitting. These models may be shown later to have a basis in more fundamental processes, but initially must be viewed as operational. The models to be discussed in this chapter fall into this latter class.
