EDIT: Apparently this argument has already been discussed (see e.g The infectiousness of nihilism by Will MacAskill). I will revise this argument and post a better version in the future.
Note: There are many forms of nihilism all with extensive literature behind them. While this method I'll be discussing can be applied to some other forms of nihilism too, I will be sticking to moral nihilism for this post.
When someone encounters a (moral) decision in life, their choice often depends on their personal philosophy. But if your dominant belief is nihilism it will tell you that it doesn't matter what you choose because there is no right or wrong answer. I say dominant belief because you can't be 100% sure that your theory is correct. This gives us some wiggle-room. Say a person is a very stout follower of nihilism with a certainty of 95%. This still leaves 5% for other theories that, while not as convincing as nihilism, have different degrees of persuasiveness to her. Let's say 95% nihilism, 3% classic utilitarianism and 2% various other theories.
At one point in her life the following event happens: There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. She is standing next to a lever. If she pulls this lever, the trolley will switch to a different set of tracks. However, she notices that there is one person on this different set of tracks.
Now we can imagine the philosophies in her head as various voices that tell her what to do with various strengths. If we take the analogy of a parliament we can say that nihilism holds 95 of the 100 seats, utilitarianism has 3 seats and the various other philosophy share the remaining 2 seats. When she consults her philosophies what to do in this situation the dominant part (nihilism) will say that there are no right or wrong answers to this situation, so it's in effect not advising anything. Utilitarianism on the other hand is strongly in favor of pulling the switch. Since nihilism is withholding it's voice, the lever gets pulled.
So in effect, nihilism is entirely irrelevant here. And since it never prescribes a preference for what action you should take, you can always disregard it. A philosophy, which says that a certain category of decisions doesn't matter, doesn't matter itself when you have to make a decision in this category.
Philosophy isn't the only motivation. If you don't have philosophical motivations to do something, it is likely that expediency or self interest will fill the gap. Expediency would tell you not to pull the lever in case you were charged with murder.
This is SO different from my experiences and observations about people's decisionmaking that I'm not sure how to adjust my models.
Disagree. Their choice is based on a very complex learning system, and their personal philosophy evolves to explain their choices. There probably _IS_ a feedback loop where repeated use of the justification encodes the decision weights more deeply, so the philosophy does appear to guide future choices. But causality definitely ain't that simple.
No. It will tell you that it doesn't _objectively_ matter. There's no right or wrong _outside of yourself_. That doesn't mean there's not preferable and dis-preferable choices, nor that you can't have personal judgements about the likely results. Nihilism doesn't deny consequentialism (nor support it; you can make choices based on your evaluation of yourself, rather than your evaluation of the consequences).
The uncertainty argument doesn't work either - Pascal's Wager fails because one can imagine a pull in every direction. There are an infinity of gods and moral theories, each of which having infinitesimal probability. Without evidence on which one(s) are more likely than others, they cancel out. If you don't believe in any specific other moral stance, you do _not_ have to act like you do just because you might be wrong.
This is probably true, but that doesn't mean you can't say that they aren't caused by their personal philosophy. Yes the learning system came first and caused the personal philosophy, but the big bang came even more first and caused the learning system. If I make a decision the outcome is based on: the big bang, my learning system and my personal philosophy. Saying that it is caused by my personal philosophy does not invalidate saying that it is caused by my learning system any more than saying that the last domino's fall was caused by the second-to-last domino invalidates the argument that it was caused by the first.
This is active nihilism aka existentialism which is distinctly different from classic moral nihilism, and thus requires different arguments (See my comments with jessicata in this thread)
This seems like an argument against Bayes theorem in general: there are infinite theories so they cancel each-other out. The answer is of course that some theories are better than others. Some theories are contradicted by the evidence, some theories are fallacies, some theories are absorbed by other theories. The same is true for moral theories, so in practice they won't be even. Plus you are only human, you'll never learn every theory and must therefore try to figure out which theories work best with the small subsection you can hold in your brain.
I'll definitely argue against Bayes theorem being used to update on non-evidence made-up scenarios.
For those of you who are wondering why I decided to quickly explain the trolley-problem: the reason is because I want you to be able to send this to non-LW friends. I myself struggled with nihilism for a long time before I thought of this argument. A less jargon filled post might make it easier to help other people put nihilism aside as well.
This argument has long been known, and much discussed. See e.g. Jacob Ross, Rejecting ethical deflationism and William MacAskill, The infectiousness of nihilism.
What you are describing is a passive sort of nihilism. Active nihilism, on the other hand, would actively try to negate the other values. Imagine a parliament where whenever a non-nihilist votes in favor of X, a nihilist votes against X, such that these votes exactly cancel out. Now, if (active) nihilists are a majority, they will ensure that the parliament as a whole has no aggregate preferences.
Your description of active nihilism seems off. As I understand it active nihilism is the philosophy that after discovering there is no inherent meaning you choose to create your own meaning. Active nihilism is kinda like existentialism. In fact the wikipedia page you link to says the same thing:
The philosophy you're describing is not nihilism but a different philosophy that preaches active indecisiveness. I personally never heard of it.
Active nihilism described in the paragraph definitely includes, but is not limited to, the negation of values. The active nihilists of a moral parliament may paralyze the parliament as a means to an end; perhaps, to cause systems other than the moral parliament to be the primary determinants of action, rather than the moral parliament.
So the very next sentence on the wikipedia page is:
And I agree that this is no longer nihilism but rather existentialism.
EDIT: The reason this matters is because you need entirely different arguments against existentialism.
As soon as there is an actual end, it seizes to be nihilism. It's not that I don't want to debate existentialism, but rather there has already been so much criticism written about it that I'm not up to date on. If you are interested in arguments against active indecisiveness I suggest you start there.
I think that the attitude you're talking about is not what is commonly understood as "moral nihilism". You are saying "nihilism will say that there are no right or wrong answers to [the question what to do]", but even the question "what should I do from a moral point of view?" is irrelevant in the moral nihilist stance.
I'd say that you're conflating the non-existence of moral reasons (that's the nihilist claims) with the non-existence of all reasons. It does not seem like being a nihilist implies that you don't have any goal, as you point out below with the discussion on existentialism. A moral nihilist (someone that holds the "meta-ethical view that nothing is morally right or wrong", from Wikipedia) could still have goals and desires upon which to act.
The various voices would not be "nothing is right or wrong" vs "pull the switch", but rather "do according to your own values" vs "do so as to maximize well-being" or even "do so as to do what is moral".
The absence of values can not matter indeed, but moral nihilism is not the absence of values rather than the denial of the metaphysical nature of these values.