Layers of Models in Computer Simulations

I discuss here the definition of computer simulations, and more specifically Hum\-phreys' (2004) views, who considers that an object is simulated when a computer provides a solution to a computational model, which in turn represents the object of interest. I argue that Humphreys' concepts are not able to analyze really successfully a case of contemporary simulations in physics, which are more complex than the examples considered so far in the philosophical literature. So, I propose to modify Humphreys' definition of a simulation. I allow for several successive layers of computational models, and I discuss the relations that exist between these models, the computer and the object under study. A consequence of my proposal is to clarify the distinction between computational models and numerical methods, and to better understand the representational and the computational functions of models in simulations.