Many Natural Law (NL) theorists center their exposition of NL around the concept of a basic good. They give lists of basic goods, such as: health, friendship, knowledge, religion, play, etc. The basic goods are incommensurable: each one provides a different aspect of fulfillment to the possessor.
An NL theorist shouldn't, however, think of the theory as depending on the concept of a basic good. For the concept is a fishy one.
The basic goods are types of goods. Types come at many levels of generality. There does not, however, appear to be a non-arbitrary level of generality at which we get the "basic goods". Let me explain.
Here is a non-arbitrary level of generality: infima species of goods, types of good that there is no way of further subdividing into further subtypes that differ qua goods. Given NL's commitments about incommensurability, one might try to characterize an infima species of good as a type of good such that (a) instances of it are all commensurable and (b) it isn't a proper subtype of another type of good with that property. The basic goods are not infima species. For instance, knowledge can be subdivided into knowledge of necessary truths and knowledge of contingent truths, and we have incommensurability between the types. Knowledge of necessary truths can then be subdivided into mathematical knowledge and non-mathematical knowledge, and again there is incommensurability there. I suspect the infima species are going to be extremely specific, e.g., Smith's intellectual friendship with Kowalska focusing on fundamental political philosophy (and it will probably be more specific than that) or Jones's knowledge of Pythagoras' Theorem on the basis of proof P17 (again, further specificity may be called for).
Here is another non-arbitrary level of generality: the highest genera. There might be just one highest genus, good. Or perhaps the highest genera are good of God and good of a creature. Or perhaps there is an infinite list of highest general but they are all instances of the schema good of N where N is a type of entity.
But the basic goods are neither infima species nor highest genera. They fall at some level of generality in between. And there seems to me to be no non-arbitrary way to delineate them. The best approach might be this: the basic goods (for humans) are the highest genera that fall properly under good of a human. (So if the good of a human is a highest genus, then the basic goods are second-highest genera.) But I doubt that there is a non-arbitrary way to define the highest genera under good of a human. There are many ways of subdividing good of a human, and the traditional subdivisions into basic goods are just one of them. For instance, one might subdivide good of a human into good of a human not in relation to other persons and good of a human in relation to God and good of a human in relation to non-divine persons (and maybe one or more hybrid categories). Or one might subdivide it into intellectual good and non-intellectual good. Etc.
Another option: an epistemic distinction. Perhaps the basic goods are the finest partition of the goods into genera with the property that one cannot fully grasp the distinctive value of any of the goods in any one genus on the basis of a grasp of the values of all the goods in the others. But I suspect that a distinction like this, if it can be made at all, would be liable to point to what is in at least some ways a finer level. Can one really grasp the distinctive value of aesthetic knowledge or friendship with Mother Teresa on the basis of other goods? Moreover, it may be that to grasp friendship one needs to grasp at least one other basic good, since friends promote each other's good not just in respect of friendship.
Fortunately, while the notion of incommensurable goods is important to NL, I do not think the NL theorist really needs a non-arbitrary concept of a basic good. The lists of basic goods are useful as heuristics, and they are a pedagogically valuable way to illustrate incommensurability. Moreover, it may be practically useful for guiding one's decisions and self-examination to have a division of goods that is sufficiently thick but not too fine-grained.