The empirical evidence that shrimp have small brains detracts from this probability, but not by much.

I very much agree that pretty much whatever our prior should be, the available evidence does not justify substantially updating away from it. I'm just uncertain about what the prior should be (see below).

I think that conditioning on p(sentience) is sufficient to justify a non-negligible probability of similar levels of welfare range in the absence of any further empirical evidence.

Yeah, agreed that's the crux! :) I think you are applying a principle of indifference (POI) across welfare subjects (or have a significant credence in such a move, at least).^[1] While I actually also have sympathy for something of the sort,^[2] it is widely criticized in the literature on cluelessness and decision-making under uncertainty. Here's a list of challenges and possible responses taken from a rough paper draft of mine on this exact topic:

• 1. Uncertainty about how to individuate welfare subjects within a "welfare-containing" entity or within a bigger welfare-containing entity this one is part of → important instance of the problem of the many. Research could help us non-arbitrarily individuate (see, e.g., Gottlieb 2022; Fischer et al. 2022; McIntyre forthcoming) but this research may face very similar challenges to that on moral weights (on how much we can update away from whatever our prior is) and not bring us far.
  • But maybe biting the bullet and accepting some arbitrariness here is the least bad option we’ve got?
• 2. Why apply POI at the level of welfare subjects or brains rather than at the level of, e.g., cells?
  • Maybe persons (i.e., welfare subjects) are themselves what is morally relevant rather than their experience moments (see, e.g., Bader 2022), but
    • we’d need a solution to the non-identity problem.
    • and an argument for why following our intuition on this is fine but not with moral weights.
• 3. Why endorse any form of POI to start with? In a complex cluelessness context like the one we’re in when estimating moral weights,^[3] the plausibility of POI is infamously contested (see, e.g., this, that, and refs therein). Hence, maybe we can’t use POI to justify a precise prior. Maybe we should favor an imprecise one such as each non-human species = (0, X). (where X = 1 or a bit higher.) (which would lead to agnosticism about whether many interspecies tradeoffs we make are justified).
  • However, to the extent that people want to reject such agnosticism (for whatever reason), even as an uninformed prior, they have to pick a precise-ish alternative prior. In this case, "everyone counts for (~)one" may be more advisable than the other options. (Wager on the possibility that we can apply POI).

A tl;dr from Claude that I like: ignorance about X's welfare range doesn't automatically justify treating X's welfare as if it equals human welfare — it might just justify suspending judgment. The move from "we don't know the ratio" to "assume the ratio is 1" needs much more justification.

1. ^{^}

   See also Dickens and Shepherd et al. (2023), who endorse this move.

2. ^{^}

   Especially as an alternative to defaulting to our intuitions or "invertebrates don't matter at all until proven otherwise".

3. ^{^}

   One could nitpick that there's technically no complex cluelessness if we're truly uninformed and ignore the (conflicting) evidence. But in that case, sure, maybe we can start with POI, but then we update towards agnosticism once we consider evidence, so the POI argument for giving everyone the same moral weight wouldn't work.

I'm more inclined towards functionalist interpretations of welfare, on which something like relative functional significance determines welfare levels. E.g. something's attention-grabbing capacity helps to determine its welfare significance. In that case, you might be deeply skeptical that small animals have the right functional role at all, but once you grant they do, it is much more plausible that welfare ranges are similar to humans.

One possibility for attention-grabbing: beings' welfare ranges may be proportional to how much attention they have to grab, and beings with richer/more detailed experiences could have more units of attention to be grabbed, with an analogy between the number of details in a visual field like the number of pixels in a computer screen. That being said, I'm not sure it's any less valid for it to be independent of the number of possible separate elements in conscious attention at a time, and I suspect it's just a matter of normative interpretation, not a matter of empirical fact.

I also think there are degrees to which something is an attentional mechanism at all or has a given functional role, that could have normative significance, and it's unlikely that there's an objective fact of the matter about how we should weigh these degrees. See my piece Gradations of moral weight, basically another two envelopes problem.

I've always found the functionalist view more intuitively compelling. The idea that experiential intensity simply scales down with the number of neurons seems hard to accept: it implies that simpler organisms live something like a barely-there flicker of experience, which also places us humans at the apex of perceived intensity in the universe. That seems to me like a sort of anthropocentrism, which could be a little suspicious.

I also think there's a distinction worth drawing between the "dimensionality" of an experience (how many qualitative states a mind can occupy) and its intensity. A simple mind might have very few "keys," but still hit each of them hard. A shrimp might have a very narrow experiential range, with little going on beyond basic valenced states, but that needn't make those states less intense. If that's right, "simpler brain" doesn't automatically mean "smaller welfare range."

This kind of sounds like you're thinking of pain as a sort of physical magnitude, like weight or charge.

I liked this chat I had with Gemini 3.1 Pro about the relationship between the intensity of subjective experiences, and the size of neuronal avalanches. You and @Bob Fischer may also find it interesting.

This kind of sounds like you're thinking of pain as a sort of physical magnitude, like weight or charge.

Yes.

I'm more inclined towards functionalist interpretations of welfare

Which kind of functionalism? I am very sceptical of at least computational functionalism (CF). Any algorithm run by a digital computer can be executed with pen and paper (although it may take a super long time), and I have a hard time imagining how such process would itself be conscious.

In that case, you might be deeply skeptical that small animals have the right functional role at all, but once you grant they do, it is much more plausible that welfare ranges are similar to humans.

This assumes that the effect on welfare of having the right functional role is not moderated, or is only very weakly moderated, by physical quantities like the number of neurons (as in Bob's book). I find this very counterintuitive. It implies that a human who is the size of a galaxy would have the same welfare as a normal human.

However, for my point to be right, I think you just need to treat these kinds of functionalist views as in the running. You don't have to be confident that they're true.

I agree. However, I think the weight of models which are practically not sensitive to physical quantities could be astronomically low. Mistakes like the one I illustrated above about gravitational force happen when the weights of models are guessed independently of their consequences. I suspect the variance in weights should not be that different from the variance in the consequences. For example, for welfare ranges of a) 10^-100, b) 10^-10, and c) 1, I would guess weights not that different from 1 on a), 10^-10 on b), and 10^-100 on c).