A recent research article from Germany has made quite a splash in the popular media and raises some very interesting questions about animal minds. The article by Daniela C. Rößler and her co-authors was published in the Proceedings of the National Academy of Sciences. The investigators studied 34 young spiders while they slept and found that there were eye movements that seemed analogous to the eye movements of human beings and other higher animals that occur during REM sleep and are associated with dreaming. They pointed out that this seems to suggest that arachnids may have mental states and dreams that are more akin to those of human beings than previously thought. The research is fascinating in its own right but I think what is even more fascinating is a question it raises about the relationship between nonhuman and human minds in the biological world. Specifically, regarding spiders, we can ask: Are spiders conscious?

That many organisms that are more primitive than human beings — such as bees and spiders and cats and dogs and apes, etc. — behave in ways that suggest the presence of mental states is undeniable. The deeper question is: Are nonhuman living things conscious, and what does it mean to be conscious? Consciousness is a surprisingly subtle concept and is in many ways rather difficult to define. Consciousness seems to correlate with alertness but there are situations (such as dreaming) in which we couldn’t really be said to be alert yet we are conscious of dreams. Research on persistent vegetative state shows that even people in the deepest level of coma are capable of relatively complex abstract thought. Thus, despite our natural association between alertness and consciousness, it seems that alertness is not really a useful marker for consciousness. The same can be said of self-awareness — very young infants do not appear to be self-aware but they are certainly conscious. There are a variety of other mental states that appear to accompany consciousness but are not definitive of it.

Defining Consciousness

In my view the most useful definition of consciousness arises from the work of Franz Brentano, who was a German philosopher in the 19th century. Brentano asked a question of fundamental importance: What property is it that absolutely distinguishes a mental state from a physical state? He observed:

Every mental phenomenon is characterized by what the Scholastics of the Middle Ages called the intentional (or mental) inexistence of an object, and what we might call, though not wholly unambiguously, reference to a content, direction towards an object (which is not to be understood here as meaning a thing), or immanent objectivity. In presentation something is presented, in judgment something is affirmed or denied, in love loved, in hate hated, in desire desired, and so on.

This property of aboutness is called intentionality and it distinguishes thoughts from physical objects. Every thought is inherently about something — I can think about my car or about justice or about the future. No physical thing is inherently about anything — physical things are just what they are. In my view, the concept of consciousness is too vague to have any useful meaning in either philosophy or neuroscience. A more precise and relevant question about animal minds would be: Do life forms aside from humans have intentionality? If they do, then they have mental states that are about something, as Brentano pointed out.

The Nature of the Mind

Aristotle was the philosopher who first thought deeply about the nature of the mind. What we call the mind today were several powers of what Aristotle called the soul, and the soul is defined as the set of abilities that living organisms have. In the Aristotelian view all living things have souls, and this is meant in a biological sense and not in a spiritual or religious sense. A plant has a soul and an insect has a soul and a dog has a soul and a man has a soul, simply by virtue of being a living thing with abilities that characterize life.

So, the question about dreaming spiders and spider consciousness is perhaps better stated as: Do spiders have intentional states, which means do they have the capacity to think about things? The presence of what appears to be rapid eye movement during sleep in spiders suggests that they may be thinking about dreams in a manner analogous to human beings. So, the question then would be: What do spiders think and dream about?

It is here that the Aristotelian distinction between the souls of animals and humans is most useful. Aristotle pointed out that animals have a range of mental powers that overlap with many of the mental powers of human beings. At least some animals have sensations (such as vision and hearing) as well as perceptions, memory, imagination, emotions, and so on, just as humans do. What animals don’t have and humans do have is the capacity for abstract thought. That is, all animal thought is restricted to particular things — to objects in their environment or to memories of particular things or to emotions that relate to particular things. What animals cannot think about is abstract concepts independently of particular things. Dogs think about food in their dish but not about nutrition. Cats think about chasing mice but not about the cruelty of a predator stalking its victim.

Perceptual and Abstract Thought

This distinction between the human capacity for both perceptual and abstract thought and the animal capacity only for perceptual thought is of fundamental importance and is the clearest distinction between human beings and other animals. In Aristotle’s lexicon, human beings are rational animals and are the only rational animals. Human beings are the only animals who can think of concepts, independently of particular physical objects. 

I believe it is undeniable that at least many lower forms of life have mental states of some sort — they have intentional mental states directed at particular physical objects. For a spider, this intentionality would likely be directed toward its web and to unwary insects trapped in it or to predators that might threaten it. These spider mental states would undoubtedly represent “consciousness” of a sort as the term is commonly understood. What distinguishes a spider mental state from human mental state is most fundamentally the capacity for abstraction, which humans have and spiders don’t.

Philosopher Thomas Nagel wrote a famous paper years ago titled “What Is It Like to Be a Bat?” Nagel pointed out that there is every reason to believe that animals such as bats (and by inference arachnids such as spiders) have mental states, but those mental states are not accessible to human beings because the human way of thinking is so radically different from the animal way of thinking that we cannot put ourselves in the place of the animal. So, the answer to the question “What is it like to be a spider?” is that we don’t know and we can’t know. That spiders do have intentional states and therefore do have minds seems undeniable — spider behavior demonstrates that there is an “aboutness” to their internal processing that seems inexplicable in the absence of some kind of mental state. And there’s no reason to doubt that spider’s mental states may also entail dreams about juicy insects trapped in the web.

The Minimal Biological Requirement

This view of intentionality and the minds of lower life forms I believe is the most realistic way to examine the question of consciousness in nonhuman life. A particularly devilish question arises now and that is: What is the minimal biological requirement to have intentional mental state? For example, no one who owns a dog could seriously doubt that dogs have mental states. But on the other hand, it seems preposterous to propose that all living things — such as bacteria and viruses — have mental states. 

So, what is the most basic requirement to have a mental state? Aristotle famously proposed that everything in the mind was originally in the senses. By that he meant that mental states are fundamentally dependent on sensory organs and that without sensory organs it makes no sense to speak of mental states. I think this is a reasonably good way to look at the question of mental states and lower forms of life — the precondition for a mental state is to have a sensory organ. Spiders, for example, have sensory organs such as eyes so it makes sense to infer that they have mental states. Viruses don’t have sensory organs and therefore it makes no sense to speak of a viral mental state.

What about Bacteria? 

Bacteria don’t have sensory organs as we commonly define them (they have no eyes or ears) but they do have the ability to sense and respond to chemical gradients, etc. Does this mean they have mental states, even though they don’t have brains or nervous systems? It seems preposterous to suggest that they could have mental states without nervous systems, but it may be equally preposterous to suggest that bacteria can “sense” chemical gradients unless we accept a mind of some sort in which “sensation” occurs. 

To assert that bacteria respond to things such as chemical gradients in the absence of mental states is to assert that bacteria are essentially tiny machines. This viewpoint was first suggested by René Descartes, who proposed that no nonhuman animal has a mind at all — but all living things below human beings are biological machines. While I find the notion that bacteria can have a mental state almost incredible, I find the description of living things as biological machines highly problematic as well. The fact is that we don’t know where on the tree of life “mind” begins. The question about bacterial “minds” will likely be unanswered for quite a while (it’s awfully difficult to ask them), and a meaningful scientific answer will await a much deeper philosophical understanding of the mind than we currently have. Perhaps Aristotle’s approach is wisest: instead of speaking of “minds” or “consciousness,” it’s best to stick to facts. What we commonly call mental states are activities — capacities and actions of living things. 

Spider “dreaming” certainly seems to suggest that spiders have activities that in higher animals we would call mental states. Bacteria, as well as spiders and humans, have activities that characterize them as alive, and that may be all that we can be sure of. Per Thomas Nagel, “What is it like to be a bacterium?” is like “What is it like to be a spider?” and “What is it like to be a bat?” We just can’t know.

I think this ambiguity would be fine with Aristotle, although he would remind us that if spiders and bacteria dream, they dream of flies and chemical gradients, but not of philosophy.