It is important to distinguish the versions of ethical naturalism which have received the most sustained philosophical interest, for example, Cornell realism, from the position that "the way things are is always the way they ought to be", which few ethical naturalists hold. Ethical naturalism does, however, reject the fact-value distinction: it suggests that inquiry into the natural world can increase our moral knowledge in just the same way it increases our scientific knowledge. Indeed, proponents of ethical naturalism have argued that humanity needs to invest in the science of morality, a broad and loosely defined field that uses evidence from biology, primatology, anthropology, psychology, neuroscience, and other areas to classify and describe moral behavior.
Ethical naturalism encompasses any reduction of ethical properties, such as 'goodness', to non-ethical properties; there are many different examples of such reductions, and thus many different varieties of ethical naturalism. Hedonism, for example, is the view that goodness is ultimately just pleasure.
Ethical naturalism has been criticized most prominently by ethical non-naturalist G. E. Moore, who formulated the open-question argument. Garner and Rosen say that a common definition of "natural property" is one "which can be discovered by sense observation or experience, experiment, or through any of the available means of science." They also say that a good definition of "natural property" is problematic but that "it is only in criticism of naturalism, or in an attempt to distinguish between naturalistic and nonnaturalistic definist theories, that such a concept is needed." R. M. Hare also criticised ethical naturalism because of its fallacious definition of the terms 'good' or 'right' explaining how value-terms being part of our prescriptive moral language are not reducible to descriptive terms: "Value-terms have a special function in language, that of commending; and so they plainly cannot be defined in terms of other words which themselves do not perform this function".
When it comes to the moral questions that we might ask, it can be difficult to argue that there is not necessarily some level of meta-ethical relativism – and failure to address this matter is criticized as ethnocentrism.
As a broad example of relativism, we would no doubt see very different moral systems in an alien race that can only survive by occasionally ingesting one another. As a narrow example, there would be further specific moral opinions for each individual of that species.
Some forms of moral realism are compatible with some degree of meta-ethical relativism. This argument rests on the assumption that one can have a "moral" discussion on various scales; that is, what is "good" for: a certain part of your being (leaving open the possibility of conflicting motives), you as a single individual, your family, your society, your species, your type of species. For example, a moral universalist (and certainly an absolutist) might argue that, just as one can discuss what is 'good and evil' at an individual's level, so too can one make certain "moral" propositions with truth values relative at the level of the species. In other words, the moral relativist need not deem all moral propositions as necessarily subjective. The answer to "is free speech normally good for human societies?" is relative in a sense, but the moral realist would argue that an individual can be incorrect in this matter. This may be the philosophical equivalent of the more pragmatic arguments made by some scientists.
Moral nihilists maintain that any talk of an objective morality is incoherent and better off using other terms. Proponents of moral science like Ronald A. Lindsay have counter-argued that their way of understanding "morality" as a practical enterprise is the way we ought to have understood it in the first place. He holds the position that the alternative seems to be the elaborate philosophical reduction of the word "moral" into a vacuous, useless term. Lindsay adds that it is important to reclaim the specific word "Morality" because of the connotations it holds with many individuals.
Author Sam Harris has argued that we overestimate the relevance of many arguments against the science of morality, arguments he believes scientists happily and rightly disregard in other domains of science like physics. For example, scientists may find themselves attempting to argue against philosophical skeptics, when Harris says they should be practically asking – as they would in any other domain – "why would we listen to a solipsist in the first place?" This, Harris contends, is part of what it means to practice a science of morality.
In modern times, many thinkers discussing the fact–value distinction and the is–ought problem have settled on the idea that one cannot derive ought from is. Conversely, Harris maintains that the fact-value distinction is a confusion, proposing that values are really a certain kind of fact. Specifically, Harris suggests that values amount to empirical statements about "the flourishing of conscious creatures in a society". He argues that there are objective answers to moral questions, even if some are difficult or impossible to possess in practice. In this way, he says, science can tell us what to value. Harris adds that we do not demand absolute certainty from predictions in physics, so we should not demand that of a science studying morality (see The Moral Landscape).
Physicist Sean Carroll believes that conceiving of morality as a science could be a case of scientific imperialism and insists that what is "good for conscious creatures" is not an adequate working definition of "moral". In opposition, Vice President at the Center for Inquiry, John Shook, claims that this working definition is more than adequate for science at present, and that disagreement should not immobilize the scientific study of ethics.