Many regularities of nature are taken for granted in daily living – the rising and setting of the sun, the effects of fire in burning anyone unfortunate enough to get too near, and so on. Experience of the world, rather than logical deductive argument, has identified these regularities. Scientific investigation, especially in the physical sciences, has greatly extended and systematized awareness of regularities. Mathematical descriptions, i.e., models, have been crucial for describing and quantifying these regularities.

As when any model is pressed into service, it is important to understand which features generalize and which do not. An engineer's scale model of a building may be helpful for checking the routing of the plumbing but may give little indication of the acoustics of seminar rooms that are included in the building. In medical research, mouse responses to disease and to therapeutic agents are widely used as models for human responses. Experimental responses in the mouse may indicate likely responses in humans.

In fundamental research in the physical sciences, deterministic models are often adequate. Statistical variability may be so small that it can, for practical purposes, be ignored. In applications of the physical sciences, variability may more commonly be a serious issue. In studying how buildings respond to a demolition charge, there will be variation from one occasion to another, even for identical buildings and identically placed charges. There will be variation in which parts of the building break first, in what parts remain intact, and in the trajectories of fragments.