 To illustrate the principle, suppose we say that gold is identical to the element with atomic number 79, that all gold has atomic number 79. Then suppose one were to ask, “Why is it that all gold has the atomic number 79?” The answer would be, “Gold just is the element with atomic number 79.” This would be a constitutive explanation. According to (S), however, this constitutive explanation precludes giving a further mechanistic explanation of why gold has atomic number 79. There is no mechanism by which gold gets atomic number 79. Having atomic number 79 just is what makes gold gold.