A quotation by George Box
Remember that all models are wrong; the practical question is how wrong do they have to be to not be useful.
Empirical Model-Building and Response Surfaces (1987)
... all models are approximations. Essentially, all models are wrong, but some are useful. However, the approximate nature of the model must always be borne in mind ...
To find out what happens to a system when you interfere with it you have to interfere with it (not just passively observe it).
Use and Abuse of Regression (1966)