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*** EXCURSUS: The tree of life
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Finally, I would like to suggest an additional methodological constraint on the search for mental states: respect for the conventions of language. There are some terms in the English language that are peculiarly reserved for mental states. The choice of these terms may change over time: before Descartes, the suggestion that something mindless could sense an object or store a memory of it or have a goal, would have seemed odd, but today, we have no problems in talking about the sensor in a thermostat, the memory of a computer (or a piece of deformed metal), or the goal of a computer game. However, for terms that currently retain a mentalistic connotation, special care will be taken to make sure that they are not employed in a way that robs them of their mental content. Some minimal mental content must be specified.
In the discussion of organisms' alleged mental capacities below, I shall treat the verbs "feel", "believe", "desire", "try" and "intend", and the nouns "feeling", "belief", "desire", "attempt" and "intention" as mentalistic terms. In ordinary parlance, these intentional terms are currently used to characterise either states of a subject ("feel", "feeling"), proposed or attempted actions by an agent ("intend", "intention", "try", "attempt"), or explanations for an agent's actions ("believe", "belief", "desire").
The verb "sense" and the noun "sensor", on the other hand, are often applied to inanimate artifacts (e.g. motion detectors), although no-one speaks of these artifacts as having "sensations". The words "perceive" and "perception", on the other hand, have a more mentalistic flavour. Modern usage draws a distinction between "sensation" and "perception" in an organism: the former is usually said to arise from immediate bodily stimulation, while the latter usually refers to the organism's awareness of a sensation (Merriam-Webster On-line, 2004, definition (1a) of "sensation"). Philosophers, however, do not always adhere to this pattern of usage. It would be prejudicial to endorse these distinctions at this stage, but we should allow for the possibility that there may be organisms that can be appropriately described as having sensations while lacking perceptions.
While contemporary usage allows us to speak of artifacts as having a "memory", from which they retrieve stored information, the verb "remember" retains a distinctly mentalistic connotation: it refers not only to stored information being retrieved but also to its coming to one's mind, as a subjective state. In popular parlance, machines are never said to "remember" anything. Accordingly, in our investigation, the verb "remember" will be reserved for organisms that are said to possess mental states. The verbs "recollect" and "recall" are even more strongly mentalistic, as they signify the intentional act of bringing something back to mind.
I shall also treat "learn" and "learning" as mentalistic terms, wherever possible. This mentalistic usage is challenged by Wolfram (2002, p. 823), but I believe there is currently no verb in common use that can replace the peculiarly mentalistic flavour of "learn" in English. The word "learn" usually means "to gain knowledge or understanding of or skill in by study, instruction, or experience" (Merriam-Webster on-line dictionary, 2003). However, we should keep an open mind. According to the above definition, gaining a "skill" by "experience" is learning. In our examination of organisms' abilities, we may find that some living things, despite lacking minds, are capable of feats that can be described as the acquisition of skills through experience. In that case, we would have to call this "learning", simply because it would be a violation of our existing linguistic conventions not to do so.
I shall, for the purposes of this enquiry, treat the words "know" and "cognition" as mentalistic, unless indicated otherwise.
Although certain verbs related to action, such as "intend", "try", "attempt" and "pursue", have mentalistic overtones in contemporary usage, other verbs - e.g. "seek" and "search" - have a neutral flavour, and will be used to describe goal-oriented behaviour by organisms, without any mentalistic connotations.
An important feature of our language is that we descibe things metaphorically as well as literally. This is precisely what Dennett's intentional stance allows us to do. Dennett approvingly cites the example of a logger who told him: "Pines like to keep their feet wet" (1997, p. 45). Adopting such a stance, he argues, is not only natural, but necessary to scientific progress:
When, biologists discover that a plant has some rudimentary discriminatory organ, they immediately ask themselves what the organ is for - what devious project does the plant have that requires it to obtain information from its environment on this topic? Very often the answer is an important scientific discovery (1997, p. 45).
While I wholeheartedly approve of the use of metaphors such as "devious project" to describe things in the natural world, I would argue that if adopting a strong, agent-centred intentional stance is not only useful but essential to our understanding of how an entity functions, then it must be more than a mere metaphor for that entity: it must be literally true. At this stage, I do not wish to pre-judge the issue of whether plants really are the devious plotters that biologists like to pretend they are. The point I wish to make is that if they are not, then we need to find the most appropriate language game to describe them. I would invite the reader to consider the following three statements:
1. Water is conducive to the growth of pines.
2. Pines thrive on moisture.
3. Pines like to keep their feet wet.
The first sentence is factually accurate, but it says too little. It describes pine trees from a purely chemical standpoint. Water is also conducive to the growth of crystals, but a pine tree, unlike a crystal, is an organism, with a telos. This is the truth captured by the second statement. Because a pine tree is an organism, it thrives when its need for water is realised. However, unless there turn out to be good scientific reasons for ascribing mental states to pines, the third sentence belongs to the realm of poetry. While botanists may like to picture pines as having likes and dislikes, it does not follow that they have to use these metaphors to describe or investigate what pine trees thrive on, and what harms them. For instance, they may be able to carry out equally productive research by adopting a goal-centred intentional stance and attempting to identify pine trees' built-in goals.
As I explained in my Introduction, my approach to the discussion of organisms' mental states is a convergent one, beginning with life-forms whose internal structure is the simplest and converging towards the animals, finally stopping at those animals that can be said to have minds. Let us begin by looking at the main branches of the "tree of life".
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