Hume's law or Hume's guillotine
is the thesis that, if a reasoner only has access to non-moral and
non-evaluative factual premises, the reasoner cannot logically infer the truth
of moral statements. (Wikipedia)
I do not claim that I can infer the truth of ought
statements from is statements but I claim that it would be possible to do
it if a suitable set of is statements turned out to be true, hence that Hume’s
law is not true in the sense in which it is usually imagined to be, as a conclusion depending
only on logical argument not on factual claims.
My argument starts with intuitionism, the philosophical
position that holds that just as humans have senses such as sight and hearing
that imperfectly sense physical facts so we have a moral sense that imperfectly
senses moral facts. For a book length exposition see Ethical
Intuitionism by Michael Huemer, for a short sketch, including my
response to obvious counterarguments, Chapter
61 of The Machinery of Freedom.
The argument for intuitionism, beyond the fact that it
describes how most people feel about morality — that certain acts really are
wicked — is consistency. We believe that our physical senses are not lying to
us about physical reality because what they report usually passes all the
consistency tests we can subject them to, consistency both between information
reported to us by sight, hearing, smell and touch and between the observations
of physical reality made by different people. It is possible that it is all an
illusion — what I know about what other people perceive ultimately reaches me
through my senses, which could all be lying to me — but it is the best evidence
we have available. If moral perceptions are similarly consistent, if almost
everyone, given a sufficiently well described situation and action, will have about
the same moral response, that would be evidence that there is a moral reality
out there which we are perceiving.
Whether that situation exists, whether almost everyone has
about the same moral perceptions, is a fact of reality, a non-moral fact. Suppose
it does. One might still reject the conclusion on the basis of an
alternative explanation. Perhaps there are no moral facts, just moral beliefs,
consistent because they were produced by biological evolution hard wiring into
us beliefs that cause us to behave in ways that lead to reproductive success, or
societal evolution producing societies that indoctrinate their population into the
set of moral beliefs that that make a society more likely to survive.
alternative explanation, however, is subject to factual tests. One could
imagine evidence showing that some widely held moral belief did not contribute
to reproductive success or societal survival. I do not claim to have any such
evidence but it is at least logically possible. If it existed, and if we observed consistency across humans
of moral judgement, that would be evidence for a moral reality that humans
could perceive; their common perceptions would be evidence of moral facts just as ordinary perception is of physical facts.
Hence it is logically possible to deduce ought from is.
It may be that moral
nihilism is correct, that intuitionism and other forms of moral realism are
wrong, that the necessary facts are not true, but they could be. Hume’s law is
not a claim about what facts exist but about the logical impossibility of
deducing moral facts from physical facts and I believe I have shown that it is
I have shown the possibility of evidence, not of proof, but
that is true of all our factual beliefs. I cannot prove that the sun will rise
tomorrow or that the Earth is round, I can only offer very strong evidence for
those claims. I believe that the is-ought claim as commonly understood,
certainly as I understood it, applies to evidence for moral facts as well as to proof of moral facts.