One of the most renowned papers by an American philosopher over the past 50 years also carries one of the most memorable titles: Thomas Nagel's "What Is It Like to Be a Bat?" published in 1974. Half of philosophy is getting the questions right, and Nagel alerts us in a footnote that "the analogical form of the English expression 'what it is like' is misleading." His question about what it is like to be a bat "does not mean 'what (in our experience) it resembles,' but rather 'how it is for the subject himself.'" Nagel could have avoided all possibility of confusion by calling his paper "What Is the Bat's Experience of Being a Bat?" -- though only at the cost of a certain provocative snap.

On first reading, the amount of specifically bat-related content proves surprisingly limited -- secondary, in fact, to Nagel's driving concern with certain nagging aspects of the relationship between the brain and consciousness. Challenging the confidence of philosophers who argue that all mental phenomena can ultimately be accounted for in neurobiological terms, he insists on the stubborn peculiarity of subjective awareness. The most precise real-time record of what my neurons and synapses are doing when I stare out the window will nonetheless fail to account for my highly specific experience of remembering a tree that was cut down 20 years ago. You can probably imagine something akin to my subjective experience. The situation becomes exponentially more complex when the form and content of the experience are not human. Nagel allows that we might be able to imagine falling asleep while hanging upside down in an attic, though just barely. The world as experienced during the waking hours is another matter entirely. Most bats, the philosopher notes,

perceive the external world primarily by sonar, or echo location, detecting the reflections from objects within range of their own rapid, subtly modulated, high-frequency shrieks. Their brains are designed to correlate the outgoing impulses with the subsequent echoes, and the information thus acquired enables bats to make precise discriminations of distance, size, shape, motion and texture comparable to those we make by vision. But bat sonar, though clearly a form of perception, is not similar in its operation to any sense that we possess, and there is no reason to suppose that it is subjectively like anything we can experience or imagine.

The bat sensorium is a vivid example of what the German-Estonian biologist Jakob Johann von Uexküll (1864-1944) called the umwelt. "This term is difficult to translate into English," writes Lars Svendsen in Understanding Animals: Philosophy for Dog and Cat Lovers (Reaktion Books). "It literally means the world that surrounds an organism, and it could be described as the world as experienced by that specific animal … We can say that the 'self-centered world' describes an organism's subjective reality." Another way to put it might be to define the umwelt as that portion of objective reality accessible to an animal given its sensory capacities and its needs as an organism.

A professor of philosophy at the University of Bergen in Norway, Svendsen gives full credit to what he calls "the amateur's view of the animal" while engaging with, for example, Descartes's notion of the animal as a sort of machine -- capable of responding to stimuli but not possessing consciousness as such, which requires language. (No cogito, ergo no cogitation.)

A less extreme formulation would insist that we only have certain access to animal behavior; whatever mental phenomena (e.g., emotion, memory, intellect) we may attribute to that behavior can only be an anthropomorphic projection on our part. It is possible to advance such ideas in a public discussion but difficult to maintain them upon returning home to a pet. "The amateur is, as the word quite literally means, one who loves," Svendsen writes, "and that loving view in itself can reveal something that the distanced [view] cannot grasp."

He quotes a line from Hans-Georg Gadamer's Truth and Method: "Having a language means exactly having a way of being that is completely different from how animals are bound to their surroundings." Without disputing the point, Svendsen insists that the "complete difference" in "way[s] of being" is vital and not a barrier -- something a pet owner comes to understand and to value, and also to ponder. Adopting a dog or cat (to keep the list within the range of his title) amounts to beginning a long-term, ad hoc course of speculation over the degree and the nature of mutual understanding possible across the species divide. Not all of this is anthropomorphism. On the contrary, quite a lot of it takes the form of a lingering fascination with, say, the absolute and unbridgeable chasm between the animal's umwelt and the world depicted on the evening news.

Svendsen suggests that humans might benefit from negating our anthropomorphic vanity by experimenting with its polar opposite: theriomorphism. "Instead of giving animals human characteristics," he explains, "we do the opposite … In this attempt to become the animal, to move like one and sense the world from the animal's perspective, you will gain a greater understanding." Most pet owners engage in a certain amount of this as it is, albeit strictly at the imaginative level. I enjoy the grace of my cats' movement and rather envy the expressiveness of their tails, but I am not willing to make any theriomorphic experiments with the litter box.

Svendsen goes on to suggest that a bat's experience of the world is not so completely closed to human comprehension as Nagel makes out. "Blind humans can develop a remarkable ability for echolocation," he notes, while hang-gliding can provide some sense of what it is like to fly. Later he offers a detailed appreciation of the strangeness of the octopus, which has photoreceptors and two-thirds of its neurons in its tentacles and can navigate through a researcher's maze with skill. It feels as if he is engaging in a bit of one-upmanship with Nagel -- one that loses much of its point if the reader tries to picture a crowd of blind people maneuvering through the air at high speed, guided only by sound.

After rereading Nagel's paper this week, I noticed an article from Smithsonian.com in my newsfeed that reported the findings of researchers at Tel Aviv University:

Using a modified machine learning algorithm originally designed for recognizing human voices, they fed 15,000 calls into the software. They then analyzed the corresponding video to see if they could match the calls to certain activities … They were able to classify 60 percent of the calls into four categories. One of the call types indicates the bats are arguing about food. Another indicates a dispute about their positions within the sleeping cluster. A third call is reserved for males making unwanted mating advances, and the fourth happens when a bat argues with another bat sitting too close. In fact, the bats make slightly different versions of the calls when speaking to different individuals within the group, similar to a human using a different tone of voice when talking to different people.

That puts an entirely new spin on Nagel's inquiry. What is it like to be a bat? The experience does not sound nearly as unfamiliar as you might expect.