i am studying this article: (1) http://philsci-archive.pitt.edu/archive/00004760/ I discovery the concept of scientific essentialism from 1. Here is an introduction to the idea from 1: At this point, i would advise people to read section 3 of 1. What is found particularly amusing is it` s use of kripke like arguments involving possible worlds. definition: P is necessary if and only if it is true in all possible worlds. P is contingent iff some worlds are true, and some worlds are false. According to Kripke, the belief that "water is H2o" is a metaphysically necessary claim because what is to be the essense of water is H20. This is obvious not a priori knowable, but it is what is know as an a posteriori necessary claim. That is, you come to know it` s necessary by doing empirical experiments. Here is a cute argument. According to scientific essentialism, certain things( called natural kinds, or properties) have as their essense relations. These relations interpreted by us as laws of nature. Example is to think of mass in the actual world. Mass M according to the SE line has the relation 2) F=GM1M2/d^2, essentially. Also, according to the SE line, 2 is necessary in following sense: 3) 2 is true in all logically possible worlds that contains M, thus, 2 is metaphysically necessary. Most people would find 3 very unsatisfying because they see 2 as being rather contingent for the reason that things could be different in other possible worlds. That we can change G in 2 to G* in 2, such that "G not equal to G*" and there would be a possible world W* such that: 4) F= G*M1M2/d^2 The whole insignt of 1, and the standard kripke line is that we are not talk about mass M in W*. We are talking about something similar to mass, but not mass. We called this M*. Therefore the relation in 4 ought to be: 5) F=G* M1*M2*/d^2 Therefore, 2 is metaphysically necessary in all possible worlds. A latter point in 1 is to note that 2 is based on a back ground assumption that the word "Mass" rigidly refers. This means that the word "mass" refers to all thing that are mass-like that satisfies the relation 2 in all possible worlds. 6) If 2 is necessary, then it assumes that the word "mass" "rigidly" refers. Question: The whole point of 1 is to say 6. 1 argues that if mass M is not rigid, then 2 is not metaphysically necessary. Can any of you see why? Would a prove be too much to ask? Add: The whole point of 1 is that if SE is true, then for every metaphysically necessary law in the actual world, there is a metaphysically contingent law correspond to it( ie, a law that is true in the actual world, but might be falses in a possible world). Here is how it works: a) 2 is metaphysically necessary, because the name "mass" rigidly refers to the thing that has the properties that we called mass in the actual world. The author called this N-law. b) 2 is metaphysically contingent, if the name "mass" does not rigidly refers, but pick out something similar to mass in every possible worlds( the author called this role filling facts). In some worlds, 2 is true, and in other worlds, 2 is false. The author called these Q-laws i guess 1 is trying to say in any complete scientific theory must contains both N, and Q laws.