This is the latest post in a series on science and metaphysics; the previous post is here.
The one and only metaphysical postulate of natural science is this:
Everything has a fixed nature.
This postulate allows the study of classes or kinds or types of things with a common fixed nature. For example, it allows the study of one piece of copper to be an instance of the copper kind and representative of all copper — past, present, and future, here, there, and everywhere. These are called natural kinds. (See for example, here.)
This metaphysical postulate enables natural science to be the science of natures. The methods of natural science depend on this postulate. Without it, there would be no natural science.
But with this metaphysical postulate, natural science does not require any other metaphysics. Natural science neither affirms nor denies other metaphysical postulates.
Naturalism is the term applied to a materialistic metaphysics that denies any metaphysics beyond nature. This is not a postulate of natural science.
Supernaturalism is the term applied to a supernatural metaphysics that affirms a metaphysics beyond nature. This is not a postulate of natural science.
Creationism is the term applied to a supernatural metaphysics that affirms the creation of the universe by a transcendent being.
The limitation of natural science to one metaphysical postulate limits the scope of natural science and the explanations that are available to natural science. It is the reason that natural science is a secular discipline.
As a secular discipline, natural science cannot explore questions or suggest answers that entail or exclude a metaphysics beyond its one metaphysical postulate. Those who imply an additional metaphysical postulate are not acting as natural scientists. It is incumbent on natural scientists and science educators to maintain the self-limitation of natural science.