Hi Toby, thank you for your kind words. I might take some time to answer, but I’m happy to continue this back-and-forth (and please feel free to challenge or push on any point you disagree with).

I believe the problem we face is practical in nature: we currently lack direct access to the affective states of animals, and our indirect methods become increasingly unreliable as we move further away from humans on the evolutionary tree. For instance, inferring the affective capacity of a reptile is challenging, let alone that of an arthropod or annelid. But when you mention the caveat “even in principle,” I feel much more optimistic. I do believe that, in principle, how affect varies can be projected onto a universal scale—so universal that it could even compare affective experiences across sentient beings on other planets or in digital minds that have developed hedonic capacity.

Despite the variety of qualitative aspects (e.g., whether Pain stems from psychological or physical origins, or signals an unfulfilled need, a threat, damaged tissue, or a desire), the goodness or badness of a feeling—its ‘utility’—should be expressible along a single dimension of real numbers, with positive values for Pleasure, negative values for Pain, and zero as a neutral point. Researchers like Michael Mendl and Elizabeth Paul have explored similar ideas using dimensional models of affect, suggesting that valence and arousal might offer a way to compare experiences across species, which supports the idea of a universal scale—though they also note the empirical gaps we still face.

So, I see this challenge as a technical and scientific issue, not an epistemological one. In other words, I’m optimistic that one day we’ll be able to say that a Pain value of, let’s say, -2.456, represents the same amount of suffering for a human, a fish, or a fly—provided they have the neurological capacity to experience this range of intensities. I recognize this is a bold claim, and given the current lack of empirical data, it’s highly speculative—perhaps even philosophical. But this is my provisional opinion, open to change, of course! :)

Not at all! I appreciate the great exchange, the challenging ideas, and the opportunity to clarify any points. Please feel free to continue discussing any aspect you think needs further clarification.

Thank you, Alfredo, for raising awareness and contributing this piece on such a critical issue. As a source of profound suffering, this devastating disease demands urgent and prioritized attention from society. 

Hi Bob, thank you for this valuable comment

You’re correct that there’s an apparent tension in how we frame Pain intensity categories. We addressed this briefly in our footnote [1]. Let me confirm: these categories are human-indexed and absolute, anchored to the intensity levels humans can experience. For example, Excruciating Pain represents the maximum intensity a human might feel under extreme conditions, such as severe torture. We use this human-centric scale because it’s the only reference point we can directly access and define with precision.

This absolute scale seems to conflict with the use of indicators in the operational definitions of intensity categories (such as Pain 'taking priority over most bids for behavioral execution' for Disabling Pain). These indicators are practical proxies to estimate where a species’ experience falls on this scale. However, they are not universal and require species-specific expertise to interpret (we discuss this in the context of differences between indicators and welfare metrics, here). The tension you pointed out—between the absolute nature of the scale and the provisional nature of the indicators, especially when diverse species are considered—is real and stems from the fundamental challenge of not being able to directly measure affective states in other beings.

This brings me to your second point: yes, you’re correct that assessing Pain levels via behavior alone is not sufficient. That is why, in the Cumulative Pain and Cumulative Pleasure methods, intensity attributions use as many diverse indicators as possible —chiefly the degree of attention demanded by the experience (hence behavioral changes), neurological evidence, pharmacology (dose/type of pain-relieving drugs), evolutionary reasoning, among others—to collectively estimate placement on this scale.

Regarding the interspecific aspect the article discusses: addressing this challenge is enormous. We believe that, alongside other approaches, examining some biological constraints on how Range (maximum intensity) and Resolution (discrimination ability) manifest in the simpler neurological structures of primitive sentient organisms can provide insights into their capacity for extreme Pain, at least for this group.

Toby, I really appreciate your detailed and thoughtful feedback 

As you point out, we don’t yet have a way to assign a mathematical equivalence among intensity categories, such as saying one Pain intensity is 10x or 1000x as painful as another. But I believe somehow (most probably heuristically and roughly) minds navigate these comparisons, as they decide whether an expected (or actual) level of Pain outweighs the expected (or actual) level of another source of Pain, guiding their behavior accordingly (as I gather, this reflects the von Neumann-Morgenstern utility theorem’s principle of deriving preferences from risk trade-offs—thanks for bringing it up). That is indeed the whole biological point of having intensities of affective states: to better steer behaviors in the Benthamian direction of minimizing Pain and maximizing Pleasure (which should, overall, ultimately maximize the organism’s net reproduction). 

Moving to your example, I believe one day those equivalences among Pain intensities (and their trade-offs with Pleasure intensities) will be described (we discuss some possibilities in this other  post on potential equivalence methods), but for now, the '13x' example you gave isn’t something we think we can estimate yet. It is possible, though, for practical reasons to compare the time spent in each level of affect—which is the assumption of the Cumulative Pain and Cumulative Pleasure metrics—which has proven useful and insightful for comparing welfare across conditions.

You also raise a valid concern about our assumption that higher intensity Pain corresponds to greater 'signal strength' and requires 'additional processing units.' Note that this is a working hypothesis only. But the idea finds some support from neurobiological studies in vertebrates, where increasing Pain intensity has been found to correlate with greater activation of nociceptive pathways and broader neural engagement (in fact also demanding more energy, adding a possible physical component over the possible biological one). We extrapolate this to suggest that, in general, more intense Pain might require more neural resources—hence the mention of 'processing units.' Let me share, for what it’s worth, my personal belief in this sense: I don’t think there are significant differences between the level of Pain that a primate and a mouse can feel (despite their differences in brain sizes, and therefore in 'processing units') because the differences lie in cognitive brain systems, not the affective systems that process Pain (the affective-cognitive brain divide- see, Panksepp et al 2017 for an engaging debate on this topic). But I do believe that primitive sentient organisms (such as annelids), despite being able to experience affective states, are not able to experience Pain (and Pleasure) in the levels we (or a mouse or a bird) can. Although the great biological wonder of the sentience threshold has been crossed in these organisms, the processing power of the affective part (or parts) in these primitive sentient organisms is still rudimentary. In fact, my personal bet is that primitive sentient organisms are LiLr (as per our classification in this piece), and higher ranges and Resolution evolved as processing power kept increasing during the first millions of years after the onset of sentience (let me also suggest sharing my review of a book that explores the onset of sentience).

And yet, you’re absolutely right that this is not proven and it might be the case that an organism with a simpler nervous system might represent intense Pain with less processing energy, perhaps through a different mechanism, like a binary 'on/off' response rather than graded signals (this would be the HiLr scenario where primitive sentient organisms might experience intense Pain without distinguishing many states).

Regarding your concerns about the ethical relevance of defining Pain units in terms of signal strength or processing units, note we’re not proposing that neural metrics (e.g., energy use) should define Pain intensity (even because biological mechanisms—let alone neurological ones—rarely work in linear ways). Rather, we’re just exploring whether such physiological correlates can shed light on how affective states, including Pain, evolved in primitive sentient organisms.

This piece exposes an unsolved scientific mystery regarding the capacity of certain organisms, like mosquitoes, to experience high levels of pain. Is the brain of a mosquito endowed with the complexity required to perceive strong affective states? This question is not only crucial for the calculation of moral weights but is also a fascinating topic in itself.

The Cumulative Pain analyses assume that the range of pain intensities varies from No-Pain to Excruciating in any sentient species. This range is needed in the method to make it flexible and adaptable across diverse taxa. Nevertheless, I personally believe that the range of different intensities of affective experiences evolved to match increasing levels of behavioral options, which are only possible with greater cognitive complexity. A mosquito, with an ephemeral lifespan and very limited behavioral choices, would not have been shaped by natural selection to require a wide range of affective intensities.

If this is the case, the hedonic capacity would differ far more between mosquitoes and humans than between humans and other cognitively complex animals that need to make decisions over a much more nuanced range of choices—indexed with several levels of affective intensity. Mosquitoes, therefore, might not experience more than the lowest intensity levels of suffering—something that, if true, would actually be excellent news.

Anyhow, this is a challenge that science needs to address with urgency.

Thank you very much Robert for all the links and sources—I really appreciate it. It’s great to hear that our work on animal suffering is being considered within your quantification efforts at the Organisation for the Prevention of Intense Suffering and the World Center for the Control of Excessive Suffering.

Regarding the definition of pain, we have actually proposed one, and it has been operationally useful. We designed it to be as universal as possible while explicitly addressing the need for special attention to higher manifestations of pain:

”Pain is a conscious experience, evolved to elicit corrective behavior in response to actual or imminent damage to an organism’s survival and/or reproduction. Still, some manifestations, such as neuropathic pain, can be maladaptive. It is affectively and cognitively processed as an adverse and dynamic sensation that can vary in intensity, duration, texture, spatial specificity, and anatomical location. Pain is characterized as ‘physical’ when primarily triggered by pain receptors and as ‘psychological’ when triggered by memory and primary emotional systems. Depending on its intensity and duration, pain can override other adaptive instincts and motivational drives and lead to severe suffering”

Hi Robert,

I really appreciate your kind words. I’d be happy to discuss the topics you’re interested in—whether in a web meeting or through ongoing message exchanges here, whichever you prefer.

Your idea of an app addressing all suffering-related questions is excellent. We hope that the results from the Pain Atlas Project can serve as a valuable source of information for such an initiative. We continue working on this project—let’s see where it leads us.

I wish you great success in the idea you are putting forward to create this center and commend the inspiring vision behind it. It is clear from your biography (which I just learned from the link to your bio) that you have dedicated your life to reducing suffering—something truly remarkable.

I would just like to highlight a key point regarding the assertion that there is a lack of “standardized metrics for measuring and comparing different types of suffering.” I believe you will be glad to know that the Welfare Footprint Framework provides a universal methodology for quantifying affective states, including both pain and pleasure, in a biologically meaningful way. Specifically for suffering, this framework incorporates the Cumulative Pain Metric, which is expressed in units of time spent in varying intensities of negative affective states. This metric allows for direct comparison of different sources of suffering across conditions and interventions.

The notation tool of the Pain-Track enables detailed analysis of the temporal dynamics of suffering, grounded in evidence from diverse fields such as physiology, neurology, pharmacology, behavioral science, and evolutionary biology.

These standardized tools and metrics not only make suffering more measurable but also facilitate informed decision-making and comparisons across a wide range of contexts. For example, the Welfare Footprint Framework has been applied to quantify welfare impacts in animal production systems, guiding policy decisions and reforms. For more details, please visit www.welfarefootprint.org.