If the prescriptive "ought" is separated from the factual "is," an intellectual analysis of the real world is by definition without normative value. The naturalistic fallacy thesis -- maintaining that normative and descriptive spheres must remain separated -- is often presented in a weak sense that seems reasonable. However, only in a strong sense -- by strictly separating facts and values -- are fallacy accusations supported. We claim that this naturalistic fallacy thesis is unsound and that normative statements instead should be based on rational understanding as found in the Darwinian and social sciences. The Cartesian compromise should be abandoned, since only naturalism can provide a cogent framework for better understanding and support ethics with a solid foundation. Many people nurture values based on tradition, whim, subgroup identification etc., and they demand respect for those values. However, we can demand respect for values only when they have a rational foundation. The common belief in the thesis of naturalistic fallacy is an anti-intellectual device that shields values from rational inquiry.