While the anatomical similarities between protoctista and animals may sound suggestive, they do not establish the presence of mental states. It has already been argued that mental states cannot be identified purely by means of anatomical criteria; an organism's behaviour also has to be considered. In any case, similarity is a matter of degree: the question of where one should draw the line between organisms with mental states and those without, cannot be answered simplay by cataloguing resemblances.

Before I discuss the mental capacities of protoctista, a caveat is in order. It now appears that most of the biological diversity within the eukaryotes lies among the protoctista, and many scientists believe that it is just as inappropriate to lump all protoctista into a single kingdom as it was to group all prokaryotes together. (Prokaryotes are now classified into two domains: bacteria and archaea.) Additionally, since the group includes both one-celled and multicellular organisms, possible differences in mental capacities (if any) should be expected.

Sensory phenomena such as chemotaxis, thermotaxis (movement in response to heat), phototaxis, geotaxis (movement in response to gravity) and an ability to identify suitable mates are well-attested for protoctista (Martin and Gordon, 2001, p. 409). However, it has been argued above (see Conclusion S.5) that sensory capacities per se do not require a mentalistic explanation. If protoctista show no signs of flexibility and novelty in their patterns of responding to their environment, then there is no point in ascribing mental states to them (see Conclusion N.11).

Habituation in protoctista

Certain kinds of protoctista (especially protozoa, such as paramecia and amoebae) are recognised as being capable of being properly habituated (Abramson, 1994, pp. 106, 112, 116, 117). This is an adaptive feat which bacteria appear to be incapable of. The evidence that protoctista are indeed capable of undergoing habituation is presented and described in an Appendix.

However, while habituation is regarded by psychologists as a form of learning, habituation in its most basic form cannot be considered as "true" learning, as the organism does not modify its patterns of responding to its surroundings. Non-associative habituation, taken alone, does not constitute evidence for mental states (Conclusion S.8).

Can protoctista learn?

More interestingly, it has also been claimed that paramecia are capable of associative learning, which Abramson (1994, p. 38) defines as:

a form of behaviour modification involving the association of two or more events, such as between two stimuli, or between a stimulus and a response. In associative learning, an animal does learn to do something new or better (1994, p. 38, italics mine).

There are two broad categories of associative learning:

Classical conditioning refers to the modification of behavior in which an originally neutral stimulus - known as a conditioned stimulus (CS) - is paired with a second stimulus that elicits a particular response - known as the unconditioned stimulus (US). The response which the US elicits is known as the unconditioned response (UR). An organism exposed to repeated pairings of the CS and the US will often respond to the originally neutral stimulus as it did to the US (Abramson, 1994, p. 39). It should be noted that if the CS and US occur simultaneously, or if the CS occurs after the US, virtually no conditioning will occur. The CS needs to precede the US and be predictive of it. An animal obtains no biological advantage in learning an association between a CS and a US unless the CS can be used to predict the US.

Instrumental and operant conditioning are "examples of associative learning in which the behavior of the animal is controlled by the consequences of its actions... [Whereas] classical conditioning describes how animals make associations between stimuli, ... instrumental and operant conditioning describe how animals associate stimuli with their own motor actions ... Animals learn new behaviours in order to obtain or avoid some stimulus (reinforcement)" (Abramson, 1994, p. 151).

The biological relevance of these kinds of associative learning is discussed by Brembs (2000). He argues that classical conditioning enables organisms in the wild to associate biologically neutral stimuli with significant ones, enabling them to make better predictions about their environment, while operant conditioning reinforces behaviour that satisfies their appetites or enables them to avoid aversive stimuli (2000, p. 2).

The fact that associative learning constitutes a new, biologically advantageous way of manipulating information raises the philosophical question of whether associative learning indicates the presence of mental states, or whether it can be explained using cognitively neutral terminology. In other words, what kind of intentional stance do we require to account for associative learning?

Certainly, associative learning qualifies as flexible behaviour according to the definition we have given. It is not fixed, as the value of the output variable (i.e. the response) does not remain the same for the same input variable (stimulus). There is genuine novelty here, which cannot be treated as a temporal extension of an existing pattern of activity within the organism by introducing extra historical variables, as we did with habituation. Instead, what we see here are either new conditions for activating an existing behaviour pattern (classical conditioning), or the emergence of a new behaviour pattern (instrumental or operant conditioning). In a simple case of classical conditioning, the organism learns to respond to a new stimulus (the conditioned stimulus) in the same way as it does to an existing one (the unconditioned stimulus). This is flexible behaviour, because one of the programs governing an organism's behaviour changes over time: there is a change in the conditions under which one of its behaviour function(s) is activated. In operant conditioning, the organism acquires a new behavioural function through "trial-and-error learning". Once again, this requires a program change.

The program governing a bacterium's response to mercury does not modify itself: it receives new, pre-packaged instructions from an outside source (another bacterium). In the case of associative learning, however, the new behaviour is acquired through an internal learning mechanism. This in-built mechanism for acquiring information allows the individual to modify its response to a stimulus.

L.5 The capacity for associative learning in an organism is a sufficient condition for its being able to engage in internally generated flexible behaviour.

I do not propose to discuss the question of whether associative learning indicates the presence of mental states in this section. This issue will be addressed in the sections on worms and insects. Here, I shall limit myself to the empirical question of whether protoctista are actually capable of associative learning.

According to an oft-cited study by Hennessey, Rucker and McDiarmid (1979), paramecia are capable of undergoing classical conditioning. This study is discussed in an Appendix, where it is argued that the study fails to meet the criteria for acceptable scientific evidence, laid down in the Introduction to this thesis. The study's results have never been replicated, despite attempts to do so, and there are no other well-substantiated cases of associative learning in protoctista.

Can protoctista calculate?

In another widely reported paper (Sample, 2000 - see Appendix), slime moulds have been credited with "primitive intelligence", because of their ability to compute the shortest route through a maze. Chopped up pieces of slime mould were placed in different corridors in a maze, with food placed at the entrance and exit. The pieces grew until they coalesced, then gradually withdrew from dead ends in the maze until they formed a thick tube linking the entrance and exit. The slime mould was thus able to find the minimum-length solution between two points in a labyrinth.

Two comments will suffice here. First, what the report actually demonstrates is that slime moulds possess some computing ability. However, in this respect they are fundamentally no different from prokaryotic bacteria or viruses or non-living computational systems. This ability does not constitute evidence of mental states (Conclusion S.1).

Second, the capacity shown by the slime moulds is better described as a case of adaptation rather than true learning. The ascription of true learning requires a reproducible learning effect in an individual (Conclusion L.2). Only if experiments showed that the slime mould had a tendency to re-create their configuration after being chopped up into pieces again and placed back in a maze with the same pattern, could we say that learning had taken place. The configuration would also have to be complex and unnatural, to demonstrate learning (Albrecht-Buehler, personal email, 30 September 2003). (The presence of such a tendency could be measured by comparing the slime mould's behaviour with that of a control group.) As far as I am aware, no such tests have been performed.

As far as I have been able to ascertain, this is the extent of the evidence for learning in protoctista. Although they are capable of being habituated (unlike bacteria), there is no good evidence that they are capable of "true" learning. After reviewing the evidence, my conclusion (a tentative one, bearing in mind the structural diversity of the protoctista) is they do not appear to be capable of having cognitive mental states.