The first thing that needs to be said about protoctista and plants is that they are eukaryotes, whereas bacteria are prokaryotes. Briefly, eukaryotic cells have a nucleus, while prokaryotic cells lack one. The differences between eukaryotic and prokaryotic cells, as well as the pervasive similarities at the cellular level between protoctista, plants, animals and fungi - especially the size, internal structure and signalling mechanisms of their cells - are described at further length in an Appendix.

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.

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), and analogues of the five senses have been found in plants (Coghlan, 1998). However, it has been argued above (see Conclusion S.5) that sensory capacities per se do not require a mentalistic explanation. Nor does the fact that protoctista (Abramson, 1994, pp. 106, 112, 116, 117) and plants (Abramson, Garrado, Lawson, Browne and Thomas, 2002, pp. 174-176), are both capable of being habituated - a feat which bacteria appear to be incapable of. While habituation is regarded by psychologists as a form of learning, non-associative habituation, taken alone, cannot be regarded as "true" learning and does not constitute evidence for mental states (Conclusion S.8), as the organism does not modify its patterns of responding to its surroundings. There is no point in ascribing mental states to protoctista and plants unless they show signs of flexibility and novelty in their patterns of responding to their environment, (see Conclusion N.11).

Can protoctista and plants learn?

More interestingly, it has also been claimed that certain protoctista (paramecia) and plants (mimosa plants) 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 gexamples 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 here the question of whether associative learning indicates the presence of mental states in this section. That issue will be addressed in the case studies on worms and insects, below. Here, I shall limit myself to the empirical question of whether protoctista and plants are actually capable of associative learning.

The evidence for associative learning in protoctista is discussed in detail in the Excursus on protoctista and the Excursus on plants, where it is concluded that studies cited in support of associative learning for these life-forms fail to meet the criteria for acceptable scientific evidence, laid down in the Introduction to this thesis. Additionally, the inability to replicate claimed positive results, combined with negative results from other studies, make it prudent to doubt the occurrence of associative learning in protoctista.

Although protoctista and plants 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 dearth of learning studies) is they do not appear to be capable of having cognitive mental states.