In Journey of the Mind: How Thinking Emerged from Chaos, Ogi Ogas and Sai Gaddam write:
Thinking is the process of turning the information from sensory input into action that does something for the body. A mind requires a minimum of two thinking elements: a sensor and a doer. The simplest hypothetical mind would have one sensor and one doer. Apparently, we are not aware of such a thing on the planet. The simplest mind we are aware of is the archaea, which has two sensors and two doers. I'm not going to pretend I know anything about this, and detail isn't important for my point. The general idea is that a protein called rhodopsin changes shape when the light changes. This starts a chain of events that leads to the archaea's archaella (flagella) moving, which moves the archaea. A little more detail is found starting at 3:00 of this video.A mind is a physical system that converts sensations into action. A mind takes in a set of inputs from its environment and transforms them into a set of environment-impacting outputs that, crucially, influence the welfare of its body. This process of changing inputs into outputs—of changing sensation into useful behavior—is thinking, the defining activity of a mind.
The archaella do not move because of the impact of the photons. Nor do they move because of a brute-force chain reaction, like dominos simply banging into each other. The archaella move because a series of chemical reactions carries the information that light with certain characteristics is in a certain direction.
That is thinking.
Not everyone agrees. I can't argue with them. What an archaea does seems too simplistic to be called thinking. The series of chemical reactions is different than a brute-force chain reaction, but not by much. It's still all stimulus and response. Cause and effect.
Here are a few quotes from Feeling & Knowing: Making Minds Conscious, by Antonio Damasio.
The first of those questions concerns intelligences and minds. We know that the most numerous living organisms on earth are unicellular, such as bacteria. Are they intelligent? Indeed they are, remarkably so. Do they have minds? No, they do not, I believe, and neither do they have consciousness. They are autonomous creatures; they clearly have a form of “cognition” relative to their environment, and yet, instead of depending on minds and consciousness, they rely on non-explicit competences—based on molecular and sub-molecular processes—that govern their lives efficiently according to the dictates of homeostasis.
Life sailed forth without words or thoughts, without feelings or reasons, devoid of minds or consciousness. And yet living organisms sensed others like them and sensed their environments. By sensing I mean the detection of a “presence”—of another whole organism, of a molecule located on the surface of another organism or of a molecule secreted by another organism. Sensing is not perceiving, and it is not constructing a “pattern” based on something else to create a “representation” of that something else and produce an “image” in mind. On the other hand, sensing is the most elementary variety of cognition.
I entirely understand the reluctance many doubtless feel to classifying that as thinking. But it is a definition we can work with. And, certainly, it is the beginning of thinking. What else is the definition of thinking? Still, let's see where everyone thinks thinking is present.As we discuss life and the kinds of intelligent management that different species rely on, it becomes clear that we need to identify the menu of specific and distinct strategies available to those creatures and give names to the functional steps they constitute. Sensing (detecting) is most basic, and I believe it is present in all living forms. Minding is next. It requires a nervous system and the creation of representations and images, the critical component of minds. Mental images flow relentlessly in time and are infinitely open to manipulation so as to yield novel images.
Among the other creatures/minds Ogas and Gaddam discuss is the roundworm. Roundworm has a new thinking element, to help out its sensors and doers. These are neurons that there authors call thinkers. It has two thinkers: one activates the doers that move the roundworm forward, and the other activates the doers that move the roundworm backward. These thinkers are also connected to each other, and inhibit each other. If food is sensed ahead, the forward thinker starts its doers, and inhibits the backward thinker. If poison is sensed ahead, the backward thinker starts its doers, and inhibits the forward thinker. The stronger the signal a thinker receives, the stronger it inhibits the other thinker.
The roundworm is choosing between option. It is making decisions. Is this sufficient to be considered thinking?
Mind is the thinking. Mind is an action. If no thinking is taking place, there is no mind. In archaea, if there is no light, the rhodopsin and archaela are still present, but there is no thinking, no mind. In humans, when in dreamless sleep, or under general anesthesia, or dead, the brain is still present, but there is no thinking, no mind.
In the roundworm, we have a creature that came into existence through the processing of information that is now processing information about two different things in the environmental, is judging how good or bad those things are, comparing their merits against each other, and acting on the decision it comes to. That's a good deal of information processing, in just a roundworm.
If all of this, and more, right on up to us, is nothing more than the physical properties and processes we know from our senses and sciences, then there is no reason for any process or creature to be subjectively experiencing what is happening, or its own existence. Nothing described suggests such a thing. There is no reason to think conscious beings constructed of different materials than us would examine us, and understand that we are conscious. No neuron, or neurotransmitter, or sense, or storage system, or any other thing or process known to our sciences, suggests it.
But if consciousness is fundamental, and if information processing unites groups of particles into a conscious unit, then, as life grows more and more complex, the conscious unit is experiencing greater and greater complexity.
The more a living thing, that is, a thing built from an information processing system, processes information with its mind, the more information processing it subjectively experiences.
How many steps are between the sensor and doer in an information processing system? How does the nature of the information in an information processing system change between the sensor and the doer? How many information processing systems does an entity have? How do the information processing systems within an entity interact?
The consciousness within each particle of its mind experiences what is happening to it. And, as the different parts of the body function as one unit - the body - all of the thinking elements function as one mind, and all of the consciousness functions as one consciousness.