Sunday, July 22, 2012

Weak and strong laws

I just sent this email to a colleague.  I thought it might amuse a wider audience.  Plus the idea in it is worth thinking about (though while the weak/strong terminology originated in the dream, the basic idea has been percolating for a while):
This morning, I had a dream where you told me that for some work of
mine (I don't know what) I need to read your chapter on laws of nature
in a book you're writing.  You then gave me a copy of the chapter and
it was 500 pages long. :-)  I don't think I read it, but I did have a
look at it, and I thought that in regard to the ideas you had in that
chapter--of which none were explicit in the dream--it would be helpful
to distinguish between "weak" and "strong" laws.

Anyway, I think the distinction I was getting at in my dream was
between non-accidental generalizations that are not constraining of
the activity of finite beings and non-accidental generalizations that
are constraining of it.  So, perhaps, it's a law--a *weak* law--of
biology that all life on earth descends from some ur-organism U.  It's
certainly a non-accidental generalization.  But it's a
non-constraining law: it has no "oomph" to prevent violations.  (If
chemicals near some deep-sea vent were about to produce life, the law
wouldn't stop them.)  Likewise, it *might* be a weak law that all the
matter in existence is made from particles of types P_1, P_2, ...,
P_n.  On the other hand, the law of conservation of mass-energy is a
constraining law.

Do you think weak laws in this sense really are laws?

And do you have a chapter on laws that I should read? :-)


Heath White said...

I would think that, if there is really no obstacle to life forming at sea-vents or wherever, then the fact that all life has actually originated from U is an accidental generalization and not any kind of a law.

In what sense *would* it be a law?

Alexander R Pruss said...

One test is supporting counterfactuals.

It supports some counterfactuals:
1K(F). If there were a carnivorous kangaroo (or sentient fungus), it would be descended from U.

For if it weren't descended from U, it wouldn't count as a kangaroo (or fungus), no matter how kangaroo- (or fungus-) like it were. Maybe that's too trivial.

Is this true?

2K. If there were a carnivorous kangaroo-like entity on earth, it would have (probably?) descended from U via kangaroos.

I don't know, but I think a case could be made for it.

Alexander R Pruss said...

(By kangaroo-like, I mean some pretty beefy specification of morphology, DNA, etc., but bracketing the origins-facts that enter into concept of a kangaroo.)

James Bejon said...

For whatever it's worth, my first instinct to the question of whether such "weak" laws really are laws was to say no. My instinct was that they're the kind of thing we have in mind when we talk about, say, "the law of large numbers". But now that I think about it, I'm not sure that certain physical laws (e.g., diffusion laws) aren't simply an application of the law of large numbers. So, I'm not sure.

That said, the more interesting question, to my mind, is whether or not this guy had a chapter on laws that you needed to read.

Rems said...

Laws are descriptions and have no causal power. The law of gravity describes gravity, it doesn't cause it.

Dagmara Lizlovs said...


"This morning, I had a dream where you told me that for some work ofmine (I don't know what) I need to read your chapter on laws of naturein a book you're writing. You then gave me a copy of the chapter andit was 500 pages long . . ." I once had a dream too. I had it Easter Morning back in 2010, a month before my father passed. In that dream I was trying to solve this equation. Every time the solution was about to come to me, this horse would interrupt me, nudging me with his nose for attention and affection. That day I told my father about the dream. My father, who at this point was suffering from very advanced dementia, had this rare lucid momement. Not only was he able to tell me what the equation in my dream was, he was able to give me the solution as well. Now that's some dreamin'! What do you think?

Dagmara Lizlovs said...

"And do you have a chapter on laws that I should read?" Yes, try chapters 1 and 2 of Genesis.

Alexander R Pruss said...


Strictly speaking, you're right: the laws are the propositions. But when we talk of the constraining power of laws, we mean the constraining power of whatever aspect of reality makes these propositions be laws.


How about this line of thought?

Humean views of laws, like that of David Lewis, shortchange the constraining aspect of laws. But what these views call "laws" are typically law-like in some significant sense, even when they are not laws in some full sense. They're not mere accidental generalizations. So maybe we should say that they are "weak laws"? Or is that giving too much credit to the Humeans?

Corny said...