A bacterial cell is extremely sensitive: it can sense a chemical if even one of its sensors comes into contact with a chemical, and it can detect the change if the number of sensors in contact with a chemical increases by just one. A bacterium has sensors known as methyl-accepting chemically sensitive proteins, or MCPs, on its surface. Thre are four varieties of MCPs, allowing bacteria to track different attractants at once. MCPs generate large output signals (via a protein inside the cell, called Che A) in response to any changes that occur. The chemical sensors in a bacterial cell signal changes in the attractant concentrations, rather than absolute concentrations of the attractants. These signals alter the cell's motion.

A bacterium's memory is a consequence of the fact that its tracking system takes a few seconds to catch up with any alteration in chemical concentrations, enabling the bacterial cell to compare its present state with its state a short time ago. The number of receptors stimulated by attractive or repellent molecules (apparently this number is an average of measurements taken over a period of about one second) is "compared" with the number of receptors stimulated in the previous measurement (stored as an internal signal representing the average of measurements taken 3 seconds ago). The memory possessed by bacteria is minimal: it can store just one set of intermediate results, allowing bacteria to remember any changes in the concentration of attractant chemicals that have occurred in the past 3 seconds. We can formulate the following scope conclusion:

R.4 All cellular organisms possess some kind of memory capacity, which enables them to detect changes in their environment.

The memory possessed by bacteria gives them the appearance of purposeful movement. Bacteria have two kinds of motion: directed movement (a "run") and random tumbling. When the concentration of an attractant increases over time, sensors send chemical signals via messenger proteins within the cell, to the cell's flagella (propellers). The bacteria reduce the frequency of their random tumbling motion and prolong the directed "run" motion of their flagella, enabling them to maintain their direction of motion and keep moving towards the attractant. (There is no point in bacteria swimming in the same direction for any longer, because random Brownian motion will knock them off course anyway.) Repellents have the opposite effect: bacteria respond by increasing their random tumbling motion until the concentration starts to decrease, which triggers a "run" away from the repellent.

As the concentration of an attractant increases, the affinity of a bacterial cell's chemically sensitive proteins (MCPs) for attractants decreases, as a result of a chemical change called methylation. The cell becomes less responsive to the attractant - in other words, sensitized.

We can describe the behaviour of these bacteria in two ways. We could adopt an agent-centred intentional stance, and say that they are exhibiting "purposeful movement" and are searching for food, on the basis of what they remember. Or, we could adopt a mind-neutral, goal-centred intentional stance (the bacteria, in response to stored information, are moving along a chemical gradient toward their goal). How do we decide which stance to adopt?

A few comments are relevant here. First, the sole warrant for saying that bacteria move purposefully is that their behaviour possesses a kind of finality - in this case, intrinsic finality, as they are alive. And while the chemical basis of a bacterium's memory does not preclude it from being a mental state - our own memory has a chemical basis - the "memory" exhibited by a bacterium can be simply defined by the quantity and quality of substances currently inside the cell, some of which persist for a short time, even after the division of the cell (Kilian and Muller, 2001, p. 2). A richer, mentalistic account appears redundant.

Rodney Cotterill. Picture courtesy of Danish Technical University.

Second, the memory exhibited by a bacterium lacks an historical dimension. As Cotterill describes it:

... the creature's recent history ... determines the instantaneous magnitudes of the various molecular concentrations. No chemical record is kept of the magnitudes of the various concentrations at different times... [T]he information in its environment concerning the spatial distribution of nutrients... is merely lumped into a single number, and the bacterium's cognitive repertoire is telescoped into a single binary choice, clockwise or anti-clockwise rotation of the flagellum (2001, p. 22).

Because a bacterial cell's detector system signals changes in concentrations of molecules rather than absolute magnitudes, it might be more accurate to say that bacteria can "sense" changes over short periods, rather than speaking of bacteria as being able to "remember" the past. As we saw above, the sensory capacities of bacteria do not require a mentalistic explanation.

Finally, the crucial test that a mentalistic explanation must satisfy is that it allows us to make new or better scientific predictions. There seems to be no scientific advantage in describing the foregoing behaviour by bacteria as mindful. A mentalistic account is therefore redundant.

S.6 The existence of memory in an organism is not a sufficient ground for ascribing cognitive mental states to it.