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Spanish jumping spider Saitis barbipes with fruit fly
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Yes, Spiders Dream — But That Doesn’t Make Them Leggy People

We don’t know where on the tree of life “mind,” in the most basic sense, begins. It might include bacteria but not viruses

A recent research article from Germany, which has made quite a splash in the popular press, raises some very interesting questions about animal minds. Animal behaviorist Daniela C. Rößler and co-authors studied 34 young spiders while they slept and found that their eye movements 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 then previously thought. The article, published in the Proceedings of the National Academy of Sciences, is open access.

The research is fascinating in its own right but I think what even more fascinating is a question it raises about the relationship between nonhuman and human minds in the biological world. Specific to spiders, we can ask the question: Are spiders conscious?

That many organisms that are more primitive than human beings – such as bees, spiders, cats, dogs, and apes, etc. — behave in ways that suggest that the presence of mental states is undeniable. The deeper question here is: Are nonhuman living things conscious? What does it mean to be conscious?

Consciousness is a surprisingly subtle concept and is in many ways rather difficult to define. It seems to correlate with alertness but there are situations (such as dreaming) in which we couldn’t really be said to be alert and yet we are conscious of dreamscapes. Research on persistent vegetative state shows that even people in the deepest level of coma are capable of relatively complex abstract thought.

Thus despite the natural association we make 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. A variety of other mental states appear to accompany consciousness but are not definitive of it.

Can we define consciousness in a useful way?

In my view the most useful definition of consciousness arises from the work of German philosopher Franz Brentano (1838–1917). Brentano asked a question of fundamental importance: What property 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 judgement something is affirmed or denied, in love loved, in hate hated, in desire desired, and so on. – Stanford Encyclopedia of Philosophy

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. But 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 other than humans have intentionality? If they do, then they have mental states that are about something, as Brentano pointed out.

Do spiders have intentions?

Aristotle (384– 322 BC) 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.

He understood the soul in a biological, not a spiritual or religious sense. A plant, an insect, a dog, and a man each have a soul — simply by virtue of being a living thing whose abilities characterize life.

Extreme magnification - Jumping spider portrait, front view

So the question about dreaming spiders and spider consciousness is perhaps better stated as: Do spiders have intentional states? That is, do they have the capacity to think about things? The presence of what appears to be rapid eye movement during sleep suggests that they may be thinking in 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 many of 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.

Do spiders have abstract thoughts?

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.

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 — 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 undoubtably 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.

Can we know what it is like to be a spider?

Philosopher Thomas Nagel wrote a groundbreaking paper years ago titled “What is it like to be a bat?” Nagel pointed out that there is every reason to believe that animals like bats (and by inference arachnids like 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 said, spiders’ behavior demonstrates 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. 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.

Can bacteria or viruses have mental states?

A particularly devilish question arises now: 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.

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, apart from 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 in 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 (1596–1650), who proposed that no nonhuman animal has a mind at all — that all living things below human beings are biological machines.

While I find the notion that a bacterium 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). A meaningful scientific answer awaits a much deeper philosophical understanding of the mind than we currently have.

Stick to Aristotle — and the facts

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 — the 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 or chemical gradients, but not of philosophy.


You may also wish to read: What does it mean to say bees “feel and think”? The New Scientist reviewer is unsure that we are ready for such a radical message. Unsure? At one time, it would have been “not science!” In The Mind of a Bee, behavioral ecologist Lars Chittka makes a claim that shows that science is slowly embracing panpsychism as a successor to materialism.


Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.

Yes, Spiders Dream — But That Doesn’t Make Them Leggy People