Here are the main reasons I find it overwhelmingly likely that mammals and birds (and very likely that fish) have morally relevant subjective experiences:
Behavior. They respond to potentially-painful things in almost all the same ways humans do (except for verbally articulating their experiences in a language I understand).
Evolution. The best evolutionary rationale I can think of for why humans have subjective experiences is that that might be a good way of motivating us to avoid experiences that tend to be bad for our reproductive fitness. (Note that this evolutionary story doesn’t suggest a strong connection between sentience and intelligence. In fact, it might suggest that less intelligent species are more reliant on strong subjective experiences to learn and motivate their behavior.) That rationale would apply to almost all mobile creatures (but much less so for mostly immobile ones like mussels or plants).
Here are the main reasons I doubt that mammals, birds, or fish have morally relevant subjective experiences:
Inaccessibility. Subjective experiences are, by their nature, personal. So I can’t directly observe these experiences in others. This applies to other humans’ sentience.
Programmability. For any given behavior, I can imagine a computer program that responds to the same stimulus in the same way without having a morally relevant subjective experience. This applies to other adult humans’ sentience somewhat, but not quite as well, because we seem to have pretty similar machinery. It applies more so to babies, children, or other humans with brains that are more different than mine.
Radical uncertainty. I really don’t have a good idea of what sentience consists of, what anatomy is needed to support it, or why it evolved. This applies to other humans’ sentience.
In the end, while there are good reasons to be uncertain about the sentience of other species, these reasons are at least somewhat applicable to other humans, too. So my uncertainty about the sentience of other vertebrate species isn’t much more than an order of magnitude higher than my uncertainty about the sentience of other humans.
Here are the main reasons I find it overwhelmingly likely that mammals and birds (and very likely that fish) have morally relevant subjective experiences:
Behavior. They respond to potentially-painful things in almost all the same ways humans do (except for verbally articulating their experiences in a language I understand).
Evolution. The best evolutionary rationale I can think of for why humans have subjective experiences is that that might be a good way of motivating us to avoid experiences that tend to be bad for our reproductive fitness. (Note that this evolutionary story doesn’t suggest a strong connection between sentience and intelligence. In fact, it might suggest that less intelligent species are more reliant on strong subjective experiences to learn and motivate their behavior.) That rationale would apply to almost all mobile creatures (but much less so for mostly immobile ones like mussels or plants).
Here are the main reasons I doubt that mammals, birds, or fish have morally relevant subjective experiences:
Inaccessibility. Subjective experiences are, by their nature, personal. So I can’t directly observe these experiences in others. This applies to other humans’ sentience.
Programmability. For any given behavior, I can imagine a computer program that responds to the same stimulus in the same way without having a morally relevant subjective experience. This applies to other adult humans’ sentience somewhat, but not quite as well, because we seem to have pretty similar machinery. It applies more so to babies, children, or other humans with brains that are more different than mine.
Radical uncertainty. I really don’t have a good idea of what sentience consists of, what anatomy is needed to support it, or why it evolved. This applies to other humans’ sentience.
In the end, while there are good reasons to be uncertain about the sentience of other species, these reasons are at least somewhat applicable to other humans, too. So my uncertainty about the sentience of other vertebrate species isn’t much more than an order of magnitude higher than my uncertainty about the sentience of other humans.