There are at least two related theories in which “all sentient beings matter” may be true.
Sentient beings can experience things like suffering, and suffering is bad. So sentient beings matter insofar it is better that they experience more rather than less well-being. That’s hedonic utilitarianism.
Sentient beings have conscious desires/preferences, and those matter. That would be preference utilitarianism.
The concepts of mattering or being good or bad (simpliciter) are intersubjective generalizations of the subjective concepts of mattering or being good for someone, where something matters (simpliciter) more, ceteris paribus, if it matters for more individuals.
There are at least two related theories in which “all sentient beings matter” may be true.
Sentient beings can experience things like suffering, and suffering is bad. So sentient beings matter insofar it is better that they experience more rather than less well-being. That’s hedonic utilitarianism.
Sentient beings have conscious desires/preferences, and those matter. That would be preference utilitarianism.
The concepts of mattering or being good or bad (simpliciter) are intersubjective generalizations of the subjective concepts of mattering or being good for someone, where something matters (simpliciter) more, ceteris paribus, if it matters for more individuals.