The purpose of science is to discover laws, which are then applied to predict and explain phenomena, develop technology, and make things. This occurs through a cycle of material induction and formal deduction.
Induction consists of making observations, defining terms, and proposing postulates. Deduction consists of taking the terms and definitions from induction, possibly with auxilliary assumptions, and deducing theorems and predictions that are submitted for testing and further induction. The cycle repeats with the definitions and postulates becoming refined until a full theory results.
Natural science is the study of natural kinds of observables. The terms and postulates of natural science are natural kinds and relations between natural kinds. The metaphysical status of natural kinds is not part of natural science. Natural kinds may have been created or are simply ‘brute facts of the world’ or have an unknown origin.
For example, the natural science of biology is the study of biological natural kinds. In evolutionary biology this is reduced to one biological kind since all organisms are considered to differ in degree but not in kind. Special creation biology (aka ‘creationism’) investigates many natural kinds of organisms, which are considered to have been created. Although this is a metaphysical position, it is compatible with a biology of natural kinds that excludes metaphysics.
Naturalism is the position that there is only one natural kind called ‘nature’ (or ‘Nature’), which is the nature of the universe. Evolutionary naturalism is the most common form of naturalism, in which everything is considered to have evolved over time so that things differ only in degree, not in kind.
What may be called ‘the postulate of naturalism’ is the assertion that nature is uniform, which is not required for material induction. Naturalism is not required for science.
For further reading, see:
Zdenka Brzović, “Natural Kinds“, Internet Encyclopedia of Philosophy
John P. McCaskey, “Induction without the Uniformity Principle“, 2015.
Lucio Russo, “The Forgotten Revolution: How Science Was Born in 300 BC and Why it Had to Be Reborn“, Springer, 2004.
Mortimer J. Adler, “The Difference of Man and the Difference It Makes“, Fordham University Press, 1993.