Symbol grounding
The necessity of groundedness, in other words, takes us from the level of the pen-pal Turing Test, which is purely symbolic (computational), to the robotic Turing Test, which is hybrid symbolic/sensorimotor (Harnad 2000, 2007). Meaning is grounded in the robotic capacity to detect, categorize, identify, and act upon the things that words and sentences refer to (see entry for Categorical Perception).
To categorize is to do the right thing with the right kind of thing. The categorizer must be able to detect the sensorimotor features of the members of the category that reliably distinguish them from the nonmembers. These feature-detectors must either be inborn or learned. The learning can be based on trial and error induction, guided by feedback from the consequences of correct and incorrect categorization; or, in our own linguistic species, the learning can also be based on verbal descriptions or definitions. The description or definition of a new category, however, can only convey the category and ground its name if the words in the definition are themselves already grounded category names. So ultimately grounding has to be sensorimotor, to avoid infinite regress (Harnad 2005).
But if groundedness is a necessary condition for meaning, is it a sufficient one? Not necessarily, for it is possible that even a robot that could pass the Turing Test, "living" amongst the rest of us indistinguishably for a lifetime, would fail to have in its head what Searle has in his: It could be a Zombie, with no one home, feeling feelings, meaning meanings (Harnad 1995).
Harnad thus points at consciousness as a second property. The problem of discovering the causal mechanism for successfully picking out the referent of a category name can in principle be solved by cognitive science. But the problem of explaining how consciousness can play an independent role in doing so is probably insoluble, except on pain of telekinetic dualism. Perhaps symbol grounding (i.e., robotic TT capacity) is enough to ensure that conscious meaning is present too, perhaps not. But in either case, there is no way we can hope to be any the wiser -- and that is Turing's methodological point (Harnad 2001b, 2003, 2006).
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Symbol Grounding and Brentano's Notion of Intentionality
"Intentionality" has been called the "mark of the mental" because of some observations by the philosopher Brentano to the effect that mental states always have an inherent, intended (mental) object or content toward which they are "directed": I see something, want something, believe something, desire something, understand something, mean something etc.; and that something is always something I have in mind. Having a mental object is part of having anything in mind. Hence it is the mark of the mental. There are no "free-floating" mental states that do not also have a mental object. Even hallucinations and imaginings have an object, and even feeling depressed feels like something. Nor is the object the "external" physical object, when there is one. I may see a real chair, but the "intentional" object of my "intentional state" is the mental chair I have in mind. (Yet another term for intentionality has been "aboutness" or "representationality": thoughts are always about something; they are (mental) "representations" of something; but that something is what it is that the thinker has in mind, not whatever external object may or may not correspond to it.)
If this all sounds like skating over the surface of a problem rather than a real break-through, then the foregoing description has had its intended effect: No, the problem of intentionality is not the symbol grounding problem; nor is grounding symbols the solution to the problem of intentionality. The symbols inside an autonomous dynamical symbol system that is able to pass the robotic Turing Test are grounded, in that, unlike in the case of an ungrounded symbol system, they do not depend on the mediation of the mind of an external interpreter to connect them to the external objects that they are interpretable (by the interpreter) as being "about"; the connection is autonomous, direct, and unmediated. But grounding is not meaning. Grounding is an input/output performance function. Grounding connects the sensory inputs from external objects to internal symbols and states occurring within an autonomous sensorimotor system, guiding the system's resulting processing and output.
Meaning, in contrast, is something mental. But to try to put a halt to the name-game of proliferating nonexplanatory synonyms for the mind/body problem without solving it (or, worse, implying that there is more than one mind/body problem), let us cite just one more thing that requires no further explication: feeling. The only thing that distinguishes an internal state that merely has grounding from one that has meaning is that it feels like something to be in the meaning state, whereas it does not feel like anything to be in the merely grounded functional state. Grounding is a functional matter; feeling is a felt matter. And that is the real source of Brentano's vexed peekaboo relation between "intentionality" and its internal "intentional object": All mental states, in addition to being the functional states of an autonomous dynamical system, are also feeling states: Feelings are not merely "functed," as all other physical states are; feelings are also felt.
Hence feeling is the real mark of the mental. But the symbol grounding problem is not the same as the mind/body problem, let alone a solution to it. The mind/body problem is actually the feeling/function problem: Symbol-grounding touches only its functional component.
Note: This article is based on an entry originally published in Nature/Macmillan Encyclopedia of Cognitive Science that has since been revised by the author and the Wikipedia community
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References
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See also
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