Physical scientists often say that spiritual science stands on unscientific grounds. How can this be true when spiritual science accepts all the findings of physical science? Some firm advocates of physical science expect spiritual scientists to remain within the limits of physical scientific knowledge. They not only want others to embrace their viewpoint, but also demand to conform to their dogma, so nothing can be added to what they say.
Source (German) Rudolf Steiner – GA 56 – Die Erkenntnis der Seele und des Geistes – Berlin, 9 April 1908 (page 269-270)