Naturalistic fallacy is an expression found for the first time in Principia Ethica, a work published in 1903 by the English philosopher George Edward Moore. According to it, the concept of good which is at the basis of moral discourse is a simple concept and can not be further defined.
When you claim to identify it with some natural property, such as useful or pleasant, it falls into the naturalistic fallacy, which includes both the naturalistic ethical theories and the ethical metaphysical theories. The choice of a solution can not entirely exclude the other ones.
It is possible to escape this contradiction by adopting the intuitionistic solution by Moore for which the good is sensed as the yellow color: in this way, you will know what it is and there are no alternative solutions. Moore soon realized that his solution, by virtue of intuitionism, could lead to subjectivist drifts: he pleaded this risk by focusing on the fact that the good is absolute, it expresses an intrinsic and universal value.
In this way, any possible subjectivism is reset at the start. However, a new problem showed up: given that the good is universal, absolute and independent, which is its nature? Certainly, it cannot have an empirical nature, because if it did it would fall into the naturalistic fallacy; but neither can it be metaphysical, because otherwise you would re-awaken the metaphysical fallacy. The solution is then advanced by Moore in recognizing that ‘good’ has an ontological status equal to that of Platonic ideas and numbers, which are absolute and objective without being either empirical or metaphysical: in this sense, the ‘good’ is just as number four.
In later writings, Moore would soften his position, by arguing that the good depends on the intrinsic nature of things; in this way, he will approach Aristotelianism from Platonism... ".
In the explanation of the onset of the 'naturalistic fallacy', one moves from 'having to be' which is the term used by Kant to indicate what is required by the moral law, regardless of any condition of fact and the entire order of nature. The moral law is an expression of reason in its practical use, that is, determining the will. The duty to provide what the law says to man, be reasonable but finite, exposed then to the empirical influences of subjective motives and subjective inclinations, is expressed in the imperative form.
Therefore, the ‘need to be' indicates "the relationship between the objective laws of the will in general and the subjective imperfection of the will."
Then, since the moral imperative is not subject to any end, nor is placed by the faculty of desire, it addresses people in categorical terms, that is unconditioned, and then it is intended: "because you have to."
It is by virtue of this duty that the possibility of action properly human is deducted: not the physical possibility to act, which belongs - as Kant says – to the order of causes and effects, but it is the moral possibility to fullfil the moral law or not, that qualifies man as a moral entity. Between the world of being - that is, of what is the way it is, according to the laws of nature - and the world of 'having to be'- that is of what is required by the moral law - an absolute hiatus opens up, the same as Hume had pointed out, denouncing the naturalistic fallacy which is to take prescriptive propositions, that is related to having to be, from descriptive propositions, related to what it is .