What is a “Law”?

According to Hung, the law of nature should be universally true and both informative (empirical) and necessary. The initial requirements are both “universality” and “truth” (133). To be a law, a statement at least should be true. If the statement is wrong, such as “earth is cube shape”, then it is not a law whatsoever. Universality means a law should be applicable “throughout time and space” (133). For example, “The tree in my backyard is an oak” is not a law.

In addition, laws should be necessary and informative. Even if a statement is true, it would still be required to be necessary and informative in order to become a “law.” The two most common divisions of necessity are logical necessity and physical necessity (Hung 135). Logical necessity means that the statement can prove itself, such as the triangle has three angles. Physical necessity means there is a necessity in nature, i.e. salt dissolves in water. If a statement is uninformative but necessary, it cannot be regarded as a law. The statement “all vertebrado has vertebrate” is not a law. Actually this statement is a tautology, which does have logical necessity but is not explanatory. On the other hand, if a statement is informative but unnecessary, it also cannot be regarded as a law. The statement “all cats can climb trees” embeds certain information, but it is not necessary. There is no evidence to deny the possibility that some cats are not capable of climbing trees.

The deductive-nomological (D-N) model, also known as the covering law model, is a logical structure that uses given initial conditions and laws (“explanans”) to explain a phenomenon known as the “explanandum” (Kitchen “Explanation”). More specifically, D-N model structures are as follows: First there will be a phenomenon to be explained. For example, “these leaves turn yellow.” Then scientists will have some initial conditions, such as “these are deciduous leaves contain chlorophyll and lutein” and “the season is fall”. Later, scientists will have to find general “laws of nature”. For example, “chlorophyll will lose color (green) when they lose magnesium ion in their porphyrin structures, and the color of lutein emerge” and “the loss of magnesium ion mostly happen in dehydrate environment (such as fall season).” Finally, through the combination of initial conditions and laws, scientist can logically explain the phenomenon why these leaves will turn yellow. The logical process is that by fitting the general law(s) into specific condition(s), it generates a specific outcome. Through this process, scientists can explain a phenomenon.

Within the D-N model, the law has to be explanatory. In the Hung’s “standard view” of law, the law should be physically necessary and empirical. If not - such as “all leaves turn yellow in fall”, then the law fail to be explanatory. Therefore we cannot use this statement to fully explain the phenomenon that why “these leaves turn yellow”. As Hung argued, laws create a causal link between initial conditions and specific phenomenon (explanandum). If the law is not explanatory, then we cannot build the causal connections.