 When we define concrete computations and the vehicles that they manipulate, we need not consider all of their specific physical properties, but only those properties that are relevant to the computation, according to the rules that define the computation. A physical system can be described more or less abstractly. According to the mechanistic account, an abstract description of a physical system is not a description of an abstract object but rather a description of a concrete system that omits certain details. Descriptions of concrete computations and their vehicles are sufficiently abstract as to be defined independently of the physical media that implement them in particular cases. Because of this, concrete computations and their vehicles are sometimes said to be ‘medium-independent’ or ‘substrate neutral’.