Staph bacteria. Picture courtesy of Janice Carr/CDC via BBC.
Our ability to describe an entity's behaviour according to the intentional stance is a necessary condition for our being able to ascribe cognitive mental states to it (Conclusion I.1). The intentional stance characterises the behaviour of an organism as movement towards a goal which it has information about. For this reason, directed movement is a requirement of intentional agency:
A.3 An organism must be capable of directed bodily movements before these movements can be regarded as a manifestation of a cognitive mental state.
But does the directed movement of an organism that uses information to seek a goal qualify as intentional agency?
Case study: bacteria
As we saw above, bacteria are sensitive organisms, possessing specialised "receptors" or information-encoding devices, which are sensitive to changes in light, pressure, gravity, concentrations of chemicals, magnetic fields, temperature, signals from other bacteria, and so on (University of Utah, 2002; Meyers and Bull, p. 555). These receptors may or may not be activated, depending on the local environment. However, sensitivity per se does not warrant a mentalistic explanation, as we saw above (see Conclusion S.3). It can be explained adequately by a mind-neutral, goal-centred intentional stance.
However, common bacteria like E. coli are also capable of directed movement towards or away from the objects they sense. For instance, they swim in chemical gradients towards attractants (e.g. glucose) or away from repellents (e.g. benzoate) - a phenomenon known as chemotaxis (Di Primio, Muller and Lengeler, 2000, pp. 4 - 5). Other bacteria display phototaxis and magnetotaxis, or directed movement in response to light and magnetic fields, respectively (Martin and Gordon, 2001, p. 219).
The occurrence of directed bodily movement in bacteria (and, as we shall see, protoctista and even plants), suggests the following conclusion:
A.4 All cellular organisms are capable of directed movement.
A bacterium actually has two kinds of movement: directed movement (known as a run, where the bacterium keeps swimming in the same direction) and random motion (known as a tumble, where the bacterium randomly changes direction). The mechanism by which an increase in the concentration of an attractant (or a fall in the concentration of a repellent) triggers directed movement is described in an Appendix. The important point is that the movement of bacteria, while directed, is not in any way flexible, and hence does not warrant a mentalistic interpretation (see Conclusion F.3). This points us to a negative conclusion:
A.5 The occurrence of directed bodily movement in an organism does not provide a sufficient warrant by itself for the ascription of mental states to it.
Protoctista and plants
Are there, perhaps, more complex kinds of directed bodily movement which require a mentalistic explanation? I discuss the directed movement of protoctista and plants in an Appendix. I conclude that there does not seem to be any reason to treat the directed movements of protoctista any differently from the directed movement of bacteria. By contrast, the directed movement of plants is almost as varied and complex as that of animals, but complexity per se does not warrant the ascription of mental states (see Conclusion C.3). Unless the behaviour is flexible, we should adopt a mind-neutral intentional stance towards it (see Conclusion F.3). Nothing that I have read suggests that plant movement is flexible according to the defintion used in this thesis.
On the other hand, as Di Primio, Muller and Lengeler persuasively argue (2000, p. 10), there appears to be no inherent reason why the absence of locomotion in plants, fungi and certain animals should preclude the possibility of cognition on their part:
A.6 A capacity for local movement (locomotion) in an organism is not a requirement for its possession of mental states.