I recently came across the following piece of dialogue from Robert Pirsig's classic, 'Zen and the art of motorcycle maintenance':
After a while [John] says, "Do you believe in ghosts?"
"No," I say.
"Because they are un-sci-en-ti-fic."
The way I say this makes John smile. "They contain no matter," I continue, "and have no energy and therefore, according to the laws of science, do not exist except in people's minds."
The whiskey, the fatigue and the wind in the trees start mixing in my mind. "Of course," I add, "the laws of science contain no matter and have no energy either and therefore do not exist except in people's minds. It's best to be completely scientific about the whole thing and refuse to believe in either ghosts or the laws of science. That way you're safe. That doesn't leave you very much to believe in, but that's scientific too.
"...Modern man has his ghosts and spirits too, you know."
"Oh, the laws of physics and of logic -- the number system -- the principle of algebraic substitution. These are ghosts. We just believe in them so thoroughly they seem real.
"They seem real to me," John says.
"I don't get it," says Chris.
So I go on. "For example, it seems completely natural to presume that gravitation and the law of gravitation existed before Isaac Newton. It would sound nutty to think that until the seventeenth century there was no gravity."
"So when did this law start? Has it always existed?"
John is frowning, wondering what I am getting at.
"What I'm driving at," I say, "is the notion that before the beginning of the earth, before the sun and the stars were formed, before the primal generation of anything, the law of gravity existed."
"Sitting there, having no mass of its own, no energy of its own, not in anyone's mind because there wasn't anyone, not in space because there was no space either, not anywhere...this law of gravity still existed?"
Now John seems not so sure.
"If that law of gravity existed," I say, "I honestly don't know what a thing has to do to be nonexistent. It seems to me that law of gravity has passed every test of nonexistence there is. You cannot think of a single attribute of nonexistence that that law of gravity didn't have. Or a single scientific attribute of existence it did have. And yet it is still 'common sense' to believe that it existed."
John says, "I guess I'd have to think about it."
"Well, I predict that if you think about it long enough you will find yourself going round and round and round and round until you finally reach only one possible, rational, intelligent conclusion. The law of gravity and gravity itself did not exist before Isaac Newton. No other conclusion makes sense.
"And what that means," I say before he can interrupt, "and what that means is that that law of gravity exists nowhere except in people's heads! It's a ghost!"
The confusion that Pirsig trades upon here is that which exists between concrete objects and abstract objects. Ghosts are purported concrete objects, which fail to exist, whilst the laws of physics are abstract objects, generalisations from empirical data, which do exist. The laws of physics have no matter or energy themselves, and they do not occupy regions of space, but they exist by virtue of the behaviour exhibited by objects which do possess matter and energy, and which do occupy regions of space. Ghosts, in contrast, are not generalisations from empirical data, but purported individuals.
Whether the laws of physics exist independently of the physical world is another question altogether. There are at least three possible philosophical positions on the laws of nature. The laws can be treated as either:
1) Contingent relations between particulars.
2) Contingent relations between universals.
3) Necessary relations between universals.
Consider as an example the law F = ma. There are three properties here, the force experienced by an object F, the mass of an object m, and the acceleration of the object a.
Under the first view, called the 'regularity theory of laws', laws are just contingent relationships between individual things possessing properties.
Under the second view, laws are still just contingent relationships, but relationships between properties, treated as universals. (When philosophers refer to properties as universals, they mean that a property is a type of abstract object, which can be possessed by various objects at different times and places, but which cannot be identified with those instances.) Under this second view, the laws could still have been otherwise than they are. This is called the 'nomic necessity' theory of laws, the point being that nomic necessity is distinguished from metaphysical or conceptual necessity.
Under the third viewpoint, the laws of nature are necessary relations between properties, treated as universals. Those who support this view often argue that the laws cannot be otherwise than they are because they spring from the essential nature of the properties they relate. In other words, the essential nature of those properties includes their relationships with other properties, and these are the relationships encapsulated in the laws of physics. Under this viewpoint, the laws of nature are metaphysically necessary.
All three views accept that the laws of physics exist, but only under the third viewpoint is it possible to argue that the laws of physics could exist independently of the physical world. Even then, the contention that the laws of physics are necessary relations between properties does not entail that those relations exist independently of the physical world unless one postulates that those properties exist independently of the physical world.