The Limits of Affordances Nikolai Alksnis Introduction There is growing support for the concept of affordances the idea that an organism can directly perceive possible actions in the environment around it. The concept of affordances seemingly offers a foundation for strong forms of externalism, such as ecological, embodied or enactivist theories of cognition (Chemero 2009, Hutto and Myin 2012, Gallagher and Zahavi 2008), which explains its increase in popularity. These theories often deny mediated, or internal theories, by appealing to the existence of affordances, essentially stating that: if we can directly perceive potential actions in the world, then there is no need to posit mediation in cognition. But can the concept of affordances really provide a stable basis for these theories? This paper argues that while there may be some scope for including the concept of affordances in theories of cognition, it does not provide the basis that ecological, embodied and enactivist theories would like it to have. The limits of affordances are directly related to the limits of direct perception. Developing claims made against direct perception by Jerry Fodor and Zenon Pylyshyn (1981), I will argue that only some elements of the environment can be directly perceived. Those in favor of affordances are right to reject Fodor and Pylyshyn s blanket denial that direct perception is possible (Turvey, et. al. 1981). However, this does not mean that all of the arguments the pair introduced are incorrect. Through careful attention to what we can conclude about direct perception, it will emerge that affordances are in fact possible, but only when tied to the physical mechanisms that an organism possesses. This explicitly rules out the direct perception of learned behaviors, for these must be built upon some cognitive mechanism and thereby rely on some form of cognitive mediation. For example, while a fork could be directly perceived by anyone as pick-up-able, or move-able, being aware that a fork is a tool for eating is not an immediate response. Without some exposure to the practices of using a fork as cutlery, there is no spontaneous desire to use a fork to transport food to one s mouth; in other words, the meaning of a fork as a tool for eating cannot be derived purely by looking at one. I will defend this conclusion against two counter claims, the first being that cultural practices are directly perceivable due to their physical presence in the environment (Heft 2003), and secondly, that affordances can be nested, and that through nested affordances, we can account for learned behavior. Through dismissing these claims it will be concluded that only a limited notion of affordances is possible. This will undermine externalist theories that use the idea outside of this limited scope and show how such theories either need to be abandoned or are required to posit some other cognitive mechanism in order to account for learned behaviors. What is an affordance? An affordance is a possible action that a cognitive agent (generally an organism) could take within its immediate environment. Gibson (1979/1986) states: The affordances of the environment are what it offers the animal, what it provides or furnishes, either for good or ill (p. 127 italics original). The most common example Gibson presents is the ability to see surfaces as something an individual creature could walk on. As humans we need mostly flat horizontal 1
surfaces which afford the act of walking. In contrast, one imagines that a spider perceives both horizontal and vertical surfaces as affording mobility. According to Gibson, what the environment offers is always some type of action and this is expressed by the actions that an organism can perform; like walkability or pick-up-ability. The types of affordances are also split into positive and negative, for example, a cliff offers the negative affordance of being fall-offable while the apple has the positive affordance of being edible. Action-based perceptions can be understood as an organism encountering a range of positive and negative affordances within its environment that guide its behavior. There are three features that mark Gibson s idea as being unique from the standard theories on perception. The first is the idea that it is potential actions that are the objects of perception. This is in contrast to the more prevalent idea that it is the properties of objects that are observed: the roundness of the apple, its green color, etc. (Gibson 1979/1986 p. 134). The second is a concept of ecology. This is closely tied to the idea of perceiving action. What actions an agent will perceive are defined by its physical make-up (the nature of its sense organs and physical bodily movements) and the environment that it is found in. Again, what the spider perceives is based on its physicality which is be radically different from a human. This is an important point, for to be able to perceive actions requires an environment in which to carry them out. It is for this reason that the theory of affordances is presented as an ecological one. The possibilities of action an agent is aware of cannot be understood independently of the environment it which it is located. This takes us to the third defining feature, which is that affordances are directly perceived. This is in opposition to the prevalent view that perception is a mediated process. Mediated cognition claims that an agent becomes aware of the world (either its properties or its potentials for action) through inferences that lead to mediated objects of thought. It is via a process of inference that we can conclude the nature of the object being observed. For example, I combine the roundness of the object and its color to conclude it is an apple. This act of mediation is often portrayed as the essential component that enables cognition. By the sheer presence of a mediating object one stands-in-for the external object in the world only then is a cognitive system able to manipulate that information that leads to action. This act of manipulating mediated content is at the heart of computational and representational theories of mind (Fodor 1975, 1987, 1994). In the development of the theory of affordances, Gibson was adamant that mediated theories of perception were wrong (Gibson 1979/1986 p. 139 140). His ideas are meant to propose an alternative to the so-called Establishment, which supports the idea of representation and inference being part of perception. For Gibson, there is no middle step between cognition and action; to perceive something is to directly have access to the affordances of an object or state of affairs in the world. To see an apple is to see an opportunity to eat. The reverse is also true, i.e. to not be aware of edibility means that one could not have perceived an apple. It is also worth noting that direct theories of perception generally ground meaning not in the individual agent but in the world. In other words it is a form of direct realism. 1 However, it is a little more complicated in the case of affordances. Meaning cannot be said to be in the 1 For more on direct realism, specifically the debate around how to account for illusions or hallucinations, see Noë 2004, Roberts 2012. 2
world, separate from a perceiver, this is because different affordances are available to different organisms. Two people can stand in the same spot yet be greeted by very different action potentials. Instead, meaning involves both the environment and the agent coming together. As Gibson notes: an affordance points both ways, to the environment and to the observer (Gibson 1979/1986 p. 129). I will explore this point more below. Affordances as a whole What defines affordances are these three ideas in combination: 1) the perception of action; 2) ecological approach; and 3) that perception is direct. A common mistake when discussing affordances is to omit one of these ideas or not realize the full consequences that all three of these ideas commit to. To show why this is essential, consider a similar theory to the concept of affordances: the action-oriented theory of representations (Clark 1997, 2008, Clark and Wheeler 1999, Rowlands 2006). This theory, in its strongest guise (Wheeler 2005), states that cognition is based on action, but that these actions are in the form of cognitive representations. To move ones arm, to adjust fingers for a better grip, to take a step, all these would be action commands that can be manipulated in order to produce cognition and therefore action. This makes it a mediated theory of cognition because representations are within the cognitive system and are not spread out between world and agent. 2 Yet it is also a theory which suggests that it is actions, not properties, are the basis for cognition. From this we can see even though the theories are related in one way, they are in direct opposition over the issue of mediated cognition. If all perception is direct then there is no need for mediation. This is a point that is often lost in the debate around embodied and enactivist theories. It is not simply a handful of ideas that work in tandem but a debate in which there will be winners and losers. The concepts of affordances and action-oriented representations would need a lot of modification in order for them to be workable within one theory and even then it may not be possible. Action-oriented representations are meant to be the basic building blocks of action: reaching, grasping and so forth. These basic actions are then to be combined together to make more elaborate behaviors. If basic action is representational in nature, then there seems to be no actions let for affordances to account for via direct perception. From this we see that theories can be in opposition to the concept of affordances, even though they share in similar ideas. The theory of affordances is not simply the idea that perception is based in action, nor is it just the idea of ecology, it is these two plus a theory of direct perception. Those that treat it otherwise are mistaken. The affordance debate Much of the philosophical debate around affordances is focused on explaining how affordances are both in the world and part of an agent (Heft 1989, Michaels 2003, Reed 1996, Stoffregen 2000). The most common expression is that of a disposition, with affordances being the triggering condition in the world (an object that is grabbable) and the effectivity being the requisite dispositional response of a given agent (an agent that grabs) (see Turvey 1992). A more recent attempt that removes this duality a duality which Gibson would most likely protest (see Cutting 1982) is by Anthony Chemero with his formulation of affordances as relations (Chemero 2001, 2003, 2009). This view denies that objects have affordances and instead 2 While there is talk around the concept of distributed representation, this is often still limited to one domain, for example, being spread out within a cognitive system, see Churchland and Sejnowski 1992. 3
focuses on features of the environment coming together to offer organisms potential actions. In other words, it rejects the lock and key idea of dispositions for a more holistic view in which the agent and features of the environment come together to make up a set of affordances. This offers a more unified picture that seems in keeping with Gibson s idea of affordances pointing both ways. That said, there has been some work to show both these approaches can be worked into one general idea (Chemero and Turvey 2007). While there is no lack of trying to ground affordances in this way, there is a distinct lack of discussion by proponents of affordances in the matter of direct perception. The debate around this topic is considered to have been already held, namely in the early 1980s just after Gibson s final book. The lack of discussion appears to indicate that proponents of the affordances have concluded that it ended in their favor (for cursory mentions see Chemero et. al. 2003, Scarantino 2003). In response to the concept of affordances, those in the Establishment argued heavily against the idea that direct perception was a viable way to account for cognition. Leading this charge were Fodor and Pylyshyn (one philosopher and one psychologist who seek to discredit direct perception), while Turvey and Shaw (two psychologists) defended the idea that direct perception of affordances is a viable theory. Though the majority of the discussion takes place in two tome-like papers (Fodor and Pylyshyn 1981, Turvey, et. al. 1981), the arguments discussed are fairly straightforward. Fodor and Pylyshyn claim that there is no way to constrain the concept of direct perception that would make it a workable theory, without falling back on a notion of inference and mediation being present in cognition. The reason that direct perception needs to be constrained is due to the fact that not all features or properties can be perceived directly. For example, anyone who has encountered the varied ways in which modern bathroom taps can be turned on and off knows that how to actually use such a device is not always immediately perceivable. Whether the tap knob can be turned, pushed, pulled or otherwise is often a matter of trial and error. This is not to say that the tap does not afford being gripped in the first place, but how to move the tap to make the water come out is not always evident by visual perception alone. We can write Fodor and Pylyshyn s argument as follows: 1) It cannot be the case that all features or properties of the world can be directly perceived, so there has to be a limitation to what affordances can inform an agent about. 2) However, there is no possible constraint that can limit affordance and still be considered a direct theory of cognition. 3) Therefore, affordances cannot be part of cognition. The majority of the work in by Fodor and Pylyshyn is to give reasons why 2) is the case, and they see 1) as simply a fact. This is in opposition to Turvey and Shaw, who contest that 1) is false. By showing that there is no need to restrain what can be directly perceived the whole argument would be invalidated. A large part of how they look to discredit 1) is by pointing out that Gibson, and his supporters, are not concerned with features or properties of the world, but rather possibilities of actions. So the real question is whether all possible actions can be directly perceived. The pair then argues that there are laws that can link features in the world to an agent having an affordance. In this solution we can see the dispositional model in action, with the affordances being the features in the world that lead to the effectivities of an agent being 4
activated. I will not spend much time on the exact details presented by both sides, for those interested there are the papers themselves. Instead, I will focus on the points that show why both these conclusions are in fact wrong and how both sides misconstrue the nature of what affordances are and what they can explain. In other words, it is not the case that affordances can be constrained without denying direct perception. Nor it is the case that all affordances can be directly perceived. Through this critique it will emerge that affordances are a viable explanation of perception leading to action but only within a limited scope. What actions can we directly perceive? It is worth noting that Turvey and Shaw are right in many of their criticisms: the case presented by Fodor and Pylyshyn is generally a very poor mischaracterization of Gibson s idea and the concept of affordances generally. As mentioned, one such misconstrual is the insistence that all features and properties cannot be directly perceived, rather than talking in terms of actions. The best example of this is Fodor and Pylyshyn s recurring claim that one cannot directly perceive something such as a shoe. That is, perceiving the shoehood of an object is something that must be inferred. However, the concept of affordances simply rejects the idea that shoehood is ever perceived at all. This is because being a shoe is clearly not an action. Instead, the concept of a shoe would be composed of perceptions such as able to put a foot in, or offers protection while walking. The use of the term shoe would merely be a shortcut for the set of these relations. It is misguided criticisms like this that lead to Turvey, Shaw and their fellow authors to claim: It is not obvious that Fodor and Pylyshyn are addressing the same subject matter as Gibson and the proponents of his ecological approach. (Turvey et. al. 1981, p. 238) While this is a fair point, this incorrect characterization should not distract from the argument as a whole. We can quickly adjust the premises to reflect this mistake: 1*) It cannot be the case that all actions performable by agents can be directly perceived, so there has to be a limitation to what affordances can inform an agent about. The claim has been modified so that it refers to an actions perceived by agents, not just the set of all possible actions or movements. As affordances are meant to be tied to perceivers it does not need to include all possible actions only those that are perceived by agents. Through the correction, the argument still challenges the ideas of direct perception, however, now it is about perception of actions not features. This move actually strengthens the argument for it can no longer be dismissed as being irrelevant. The theory of affordances is in need of a way to show how direct perception is able to account for all possible perceivable actions. Why is it that when I encounter and apple or chair in the world I am, without inference, able to know that one is edible and the other I sit-on-able? To account for this Turvey and Shaw appeal to laws: The perceiving of X-having-a by Z presupposes a law L (Turvey and Shaw et. al. 1981, p. 283, italics mine). Here X is a feature of the world, a is the affordance it offers and Z is the agent perceiving a. By explaining this relation as being a law they look to strengthen the link between an agent encountering an object and knowing the possible actions that can be taken. To illustrate this they present several examples, the clearest being how sharks use electric fields around their bodies in order to find 5
things to eat. This is particularly useful to detect flatfish (common types being halibut or flounder) that lie in sand or mud in order to avoid detection. When the shark swims over such a fish, their electric field is disrupted by the flatfish s own bioelectric field and the shark is then aware that food is present. The point of the example is this: if we are able explain how the shark s field is disrupted by law-like relations that is a scientific account for the physical processes going on then this exact same law can be used to account for how the shark is aware of there being food in a certain location. The fact of the matter that the fish has a bioelectrical field of its own is called an invariant by Gibson, and his theory of perception is based on being able to pick-up (directly perceive) invariants in the world (Gibson 1966). By being attuned to such invariants an organism can behave, or potentially behave accordingly (see Scarantino 2003). This emphasis on invariants is a vital part of Gibson s theory as it fills two roles. First, it overcomes concerns about variation in perception. When I encounter my office chair I can see it from many angles. Primarily I would I encounter it from the back, but I could have left it rotated, so I encounter it from the side. Or maybe it has been moved so I see it in another location from the front. In all cases I am easily able to identify it as my office chair even though the perceptual experience of it can be different each time. 3 An appeal to some invariant quality of objects or states of affairs is how Gibson explains direct perception is possible even with the different perspectives we may have at any given time. The second role of invariants is to fully remove any need for inferences. If there are specific features of the world that are present, unchanging, and can be perceived, then an organism does not need to make any inferences. It simply accesses the invariants in order to know how to act. In other words, by developing a way to become attuned 4 to the invariants, the organism would have all the information it needs to move about and survive in the world. In other words, there is no need to make an inference when the world itself presents all the information. This is a similar point to the roboticist Rodney Brook s claim that the world is its own best model (Brooks 1991). The concept of an affordance adds to this by limiting what an individual creature can do and be aware of in the world to its own physicality. This then defines the kind of knowledge a creature can have, as it is would be restricted to actions, to its own physicality. This offers a reading of Gibson that is very close to phenomenologists such as Merleau-Ponty (see Sanders 1993) and it is through this approach that many cognitive scientists think intelligence and cognition can be explained (such as Haugeland 1998 and Varela et. al. 1991). Limits of affordances While invariants present a way to explain acts of direct perception, the question is still whether they can account for all perception and action. It is the concept of invariants that particularly troubled Fodor and Pylyshyn and why they claimed that direct perception is not a viable cognitive explanation. On this matter I think they are correct. What proponents of affordances are doing is confusing cases of hardwired physicality of perception to action, with other cases of learned or more flexible cognition. For example, invariants do seem to be a good way to account for such things as the shark s awareness of food. It may also apply to cases such 3 Of course Husserl spent much time on this specific issue (Husserl 1913/1982). 4 Gibson s term. 6
as how baby turtles are able to find the sea when they hatch on the beach, 5 or explain basic creatures such as Sphex wasps with their purely preprogrammed behavior (see Dennett 1984, Hofstadter 1986). But what about the cases where there isn t this instinctual, almost mechanistic behavior? In these cases it seems that the information being perceived will simply not be able to do the job that is required (Fodor and Pylyshyn 1981, p. 143), in terms of linking invariants to an agent s awareness of potential action. To illustrate this I return to those peskily designed modern taps. In this scenario, let's consider two identical taps: silver cylinders that have no markings to indicate which way they should be moved in order for water to come out. For the designer's pure enjoyment these taps do not activate in the same way. When facing towards the taps, the one on the right needs to be moved up and down, while the left hand one needs to be moved side to side. As the taps are identical, it cannot be the case that on perceiving the tap there is any indication of which way it moves. In other words, there is no law as there is with the shark that leads to a certain response given certain ambient energy. The trial and error that will be needed to activate the taps is not a case of direct perception and would need to be explained separately from the notion of affordances. To counter this, a proponent of affordances may want to protest the fact that the taps can be moved up and down or side to side and that is what is being perceived in both cases. In other words, there are action potentials there, but the appropriate one is not apparent. This may be true, but whether there is truly an invariant law that is being perceived, I am not so sure. However, the more important issue is that once I have learnt to manipulate the tap in the correct way, what has changed about my perception/cognition when I encounter the taps again? It cannot be said that I have homed into some invariant of the left tap versus the right tap, for surely their position cannot be linked to a law that states one can be moved in a certain way. Clearly my perception/cognition of the taps has been mediated by some other or some combination of processes. Even if we are generous and grant that the location of the tap somehow adds to the attunement I have, there are still concerns. The act of attunement means I will be aware of different potentials for action when positioned in front of either of the taps. Its location should provide perceptual information to guide my behavior. However, let's say that the malevolent designer of modern fixtures has decided to switch the taps, meaning the right one goes side to side and the left one up and down. Luckily, I am informed of this by a friend as I walk into the bathroom. Through a very different form of ambient energy 6 speech compared to the visions of seeing the taps I then go on to behave based on this information. Without hesitation I move the right tap up to the side, successfully making water flow from the tap. This is an act which is in total disregard to any attunement, that is, any perceptual information I have in terms of position in relation to the tap is not guiding my behavior. The idea of inference and cognitive manipulation appears essential in this example. More so, this example shows that it is not the coming together of the environment and the agent that leads to certain actions. Rather, the 5 Light reflecting of the water plus the gradient of the beach are both meant to play a role in guiding the infant turtles to the sea (Bolten and Witherington 2003). 6 Ambient energy being the raw information that a perceiving agent has in contact with their senses at any one time. 7
situational information presented by the environment has stayed the same. Something about me has changed and this act of change suggests some internal cognitive processes being involved. From this, we can conclude that the concept of affordances does seem to account for all cognitive/behavioral actions but that it is severely limited. The so-called animats that Rodney Brooks and others designed could be understood to be using affordances in order to mimic animal behavior. Consider the robot female cricket that can move towards a male cricket s mating call with great success (Webb 1994, 1996). Invariant features of the male cricket s chirp are used such as the frequency of the call, the specific pattern of the call to guide the behavior of the both the biological and artificial female crickets. There is also no inference occurring, as the mechanisms guiding the crickets behavior are built into the very physical structure of the cricket itself. However, the cases in which there is linking from invariants via perceptual mechanisms to action is only a part of human and animal cognition. Added to this, such thinking is easily criticized for being overly mechanistic (see Dreyfus 2007). This reliance on physicality fails to explain flexibility and adaptability in cognition and behavior. So while we have found a way to explain how direct perception of affordances is possible, namely the detection of invariants, it is one that severely limits their role in the cognitive story. Defenses for unlimited affordances Those unfamiliar with the discussion of affordances might readily accept this limitation that ties the concept to innate, almost mechanistic behavior. However, this is not how the term is used in most of the literature. Instead, it is often presented as a much more encompassing idea (see Chemero 2009, Gallagher and Zahavi 2008). What is common throughout such uses is how readily, yet with questionable justification, they slip between discussing mechanism-like responses to more advanced intellectual capabilities. An illustration of this is in John Sanders otherwise excellent essay on the similarities between Merleau-Ponty and Gibson (Sanders 1993). He starts his description of affordances by stating: [P]rimitive organisms, [ ] react to their environments in terms of opportunities and dangers that are presented to them. When hungry, an organism explores its environment [ ] and seeks out opportunities for relief of the hunger. (Sander 1993, p. 290-291) It is not two paragraphs later that we are informed that: For intelligent species, [ ] it is not inconsistent with this analysis to argue that some affordances may vary from culture to culture, or from society to society. (Sanders 1993, p. 291) What is missing from this, and almost all expressions of affordances, is just how to go from the primitive organism to the more complex species. Why do we need such an explanation? Because the processes that lead to a physical trait of an organism being created, namely evolution, and ones that are developed through trial and error and other forms of learning, are hugely different. In other words, the reasons and justification we can use in the shark case are not the same we can use in the case of the taps. There is a large difference between how the agents receive information about the action potentials in their environment. Even if it is suspected that cases like the shark are the evolutionary precursor of cases like the taps, a detailed account of how this has come about is required. 8
A suggested way to counter this conclusion is an appeal to social practices and how they physically change the world around us (Heft 2003). While learned behaviors may not develop as evolved ones do, the social constructions we have instead change and inform us in the same way. However, this just exposes how much is left unexplained by the concept of affordances. To illustrate this, consider a contingent, culturally driven act we have of using forks to eat with. How do we know to use forks in a certain way? We can grant that a fork is pick-up-able and a good size for moving food to our mouths with. These could all be affordances we directly perceive from the object itself. The question is how to we go from these fundamentals to the actual act of using cutlery to eat with. A cultural account of affordances would want to include things like: the fact it is found in restaurants and kitchens; that there is a long history of fork usage; that an individual will encounter others using it as cutlery etc. These are all good reasons for how fork usage changes the physical landscape around us but it gets us no closer to understanding the link between specific invariants that forks presents and its action potentials. Being present on a dining room table can hardly be said to be an invariant feature of the world. Added to this, if we want to include a concept of learning and exposure over time, this undermines the notion of direct perception. Such ideas are reliant on a theory of associations or similarity. Using another theory such as an associationism may be an acceptable move to make thought it is outside the framework of direct perception. Oddly, Gibsonians are hostile to the idea that affordances can be coupled with something like a theory of associations (Barrett 2011). Gibson offers his own solution for how complex action can arise out of simple perceptions. For this he uses the idea of nested affordances (Gibson 1979/1986). Unfortunately, this is not a fleshed out idea, Gibson and others spend little time on the topic. What can be said about nested affordances is that it is a grouping together, perhaps hierarchically, of affordances in order to account for complex behavior (Stoffregen 2003). Unfortunately, this does not solve the issue at hand. It may be the case that several responses to invariants can be grouped together to elicit a complex action. However, this does not account for how an agent becomes aware of variable, contingent actions. I may be aware the fork is not easily bendable by looking at it, that it can be picked up, and so on and all these can be combined together. Yet this takes me no closer to knowing how I should use the object, what actions it affords me. There is simply no direct correlation in these types of cases between the information received and the act that a person takes. There needs to be something more, something else that allows for a person to perceive that the objects in front of them can be used for complex social actions. Conclusion Why I think this limitation on affordances goes unnoticed by Gibsonians is due to confusion about how a system becomes attuned to its environment. What Gibson seems to be appealing to is ideas of forming routines, habits or practices that are intimately tied with invariants but allow for contingent behaviors. This would make the theory of affordances even more like Merleau-Ponty s ideas of embodiment and habit (see above). However, by denying internal cognitive mechanisms Gibsonians are left with no way to account for the development of novel behavior. Contrast this to Hubert Dreyfus use of Merleau-Ponty to develop the notion of skilful coping (Dreyfus 2002a, 2007). In order to develop skills such as driving a car or playing chess, Dreyfus suggests that we in fact do use representations as part of the learning process, casting off the representations once the skill has been developed to a certain level (Dreyfus 2002b). So while a chessboard does not afford that a knight must move in the unique way it 9
does, being able to use other cognitive processes such as representation allows such a contingent behavior to be associated with the particular piece. The theory of affordances, with its over reliance on direct perception of invariance, lacks the ability to explain these cases. With this we see some of the problems with the notion of affordances, yet also reveal how it could be part of the cognitive picture. Affordances do seem a viable way to explain aspects of cognition when invariants can be linked, through a law-like way, to behavior. This shows that Fodor and Pylyshyn s argument against the very idea of affordances is false. However, this concept of affordances leaves out a large portion of cognitive and behavioral activity when this intimate link between physicality and invariants is not present. This is an important point, as it undermines many uses of affordances by ecological theories. At the very least, more work needs to be done to either connect affordances to other types of cognitive activity, such as a representational theory, or new accounts for the nature of direct perception need to be offered. References Barrett, Louise (2011). Beyond the brain: how body and environment shape animal and human minds, Princeton, New Jersey: Princeton University Press Bolten, Alva B. and Witherington, Blair E. (2003). Loggerhead sea turtles, Washington, D.C.: Smithsonian Books Brooks, Rodney (1991). Intelligence without representation, Artificial Intelligence, 47, 139 159 Chemero, Anthony (2001). What we perceive when we perceive affordances, Ecological Psychology, 13, 111 116 Chemero, Anthony (2003). An Outline of a Theory of Affordances, Ecological Psychology, 15:2, 181-195 Chemero, Anthony (2009). Radical Embodied Cognitive Science, Cambridge, Massachusetts: MIT Press Chemero, Anthony and Turvey, Michael T. (2007). Gibsonian affordances for roboticists, Adaptive Behavior, 15, 473 480 Chemero, Anthony, Klein, Colin, and Cordeiro, William (2003). Events as Changes in the Layout of Affordances, Ecological Psychology, 15:1, 19-28 Churchland, Patricia S. and T. Sejnowski (1992). The computational brain, Cambridge, MA: MIT Press. Clark, Andy (1997). Being there: Putting brain, body, and world together again, Cambridge, MA: MIT Press 10
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