It is not my intention to speak about the use of models in scientific inquiry or the connections between formal systems, theories and models. The problem I should like to discuss can be loosely stated as follows: Do our scientific books and articles contain descriptions of nature; does the scientist construct a picture or model of nature? Or again: Is our scientific knowledge true if, and only if, it is a model of nature? Is, for instance, Hertz right when he states ‘We make for ourselves internal pictures of external objects… When on the basis of our accumulated previous experiences we have succeeded in constructing pictures… we can quickly derive by means of them, as by means of models, the consequences…’ — or must we not only believe Dirac when he states: ‘… the main object of physical science is not the provision of pictures’ but go one step farther and say that knowledge of nature has no more to do with a model of nature than religion has to do with idols? Or to put it boldly: does the scientist observe the first part of the third Commandment: ‘Thou shalt not make unto thee any graven image, or any likeness of anything that is in the heaven above, or that is in the earth beneath, or that is in the water under the earth…’? And if this should be the case: in what sense can knowledge be true?