We know that other humans have inner lives much like our own: their expressions of joy and sorrow reflect genuine experiences, and these experiences merit our moral concern.
At least some nonhuman animals also have inner lives. Even if it is difficult to imagine what it is like to be a western lowland gorilla or a bottlenose dolphin, we can be reasonably confident these animals are capable of experiencing their own pains and pleasures, and we should afford those experiences the moral interest they deserve.
Of course, we shouldn't expect all living things to possess an inner life. There may well be nothing it is like to be a bacterium. Understanding where the line lies between those creatures who consciously experience the world and those who do not is a difficult but important problem. According to many ethical theories, our moral duties extend to all animals who are capable of experiencing pains and pleasures.
This series of posts introduces the reader to the topic of invertebrate sentience, which is the question of which (if any) invertebrates have inner lives. Invertebrates are farmed in the trillions, and more than 99% of all wild animals are invertebrates, so determining whether invertebrates merit our moral concern is a pressing task. The included posts do not definitively settle the matter (though the final post offers tentative probabilities), but they do offer a framework for thinking about the issue.