Observations 1)A very rich class of systems(but by no means all systems of interest to us )are described by differential and difference equations 2)Such an equation, by itself, does not completely describe the input-output behavior of a system: we need auxiliary conditions (initial conditions, boundary conditions) 3)In some cases the system of interest has time as the natural independent variable and is causal. however. that is not always the case 4)Very different physical systems may have very similar mathematical descriptionsObservations 1) A very rich class of systems (but by no means all systems of interest to us) are described by differential and difference equations. 2) Such an equation, by itself, does not completely describe the input-output behavior of a system: we need auxiliary conditions (initial conditions, boundary conditions). 3) In some cases the system of interest has time as the natural independent variable and is causal. However, that is not always the case. 4) Very different physical systems may have very similar mathematical descriptions