From my blog on Natural Law, The Road to Concord:
A fundamental principle of Natural Law is that definitions are fixed. Once a thing is defined, it does not change. If it does, then it is not the definition that has changed, but the thing itself, in which case, the definition is still fixed, it just no longer applies to that thing. While this may sound confusing, it is not. And while it may not seem important, it is imperative to everything we do. Please allow me to explain.
We all understand ‘dog’ to mean or refer to a member of the ‘canine’ family. The term ‘dog’ then refers to all dogs, everywhere, at all times — past, present and future. But there are those who would object along lines that might go something like this:
“If you go back far enough in evolution, eventually, you will find a step in the evolutionary ladder where it is not a dog.”
Those who would object along these lines often expect such statements to be accepted as proof that ‘dog’ does not necessarily have to refer to a member of the canine family — but they are wrong! You see, by their own admission, the animal that preceded the dog in their evolutionary ladder was something other than a dog. This means the term ‘dog’ does not apply to that animal because that animal is not a dog. Unfortunately, it also means that the people making such objections do not understand the rules of reason, either.
This principle does not work in reverse. Just because I apply the same term to a thing, that does not mean the word defines that thing equally in all cases. Sticking with our word, ‘dog,’ suppose I said, “My car is a dog.” This is a colloquialism which is usually understood to mean my car does not run well: it has little power or pick up when I press on the gas pedal. But in no way does “My car is a dog” mean that my car is a member of the canine family.
So, where a definition is fixed, the definition does not depend on the word we assign. Nor does the word define that definition. In all cases, a definition is fixed by a things form and function. Once this definition is fixed, it can be changed unless and until additional understanding of that thing’s form and function is discovered. At which time, the definition isn’t so much changed as it is modified. For example: if I say that a certain animal is a dog, and upon further study, I discover it is a schnauzer, I have not changed the fact that it is still a dog, I’ve just modified our understanding of this particular animal by using a term that inherently includes more information specific tot he animal. Inherent in the word schnauzer is the understanding that it is still a dog, but a dog with certain unique characteristics. So the definition did not change, I just added more information.
These principles must be fixed. They cannot change. If they do, then language becomes meaningless. If a word only has meaning to the person reading or hearing it, then the words I am writing could mean anything at all. This would be akin to claiming that the laws governing motion are meaningless. Only, in this case, since we are dealing with something that is tangentially related to morality, we can (and often do) ignore these laws. The result is always the same: lawlessness. But people still insist on violating them. However, because they are laws, and they are fixed (by definition), these laws do not change. All that changes is the lives and societies of those who ignore or disregard them. When this happens, eventually, that individual goes mad and the society collapses.