action is the ability to do the action or to tell if it has been done." Consciousness 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 entries for Affordance and for Categorical perception). On the other hand, if the symbols (words and sentences) refer to the very bits of '0' and '1', directly connected to their electronic implementations, which a (any?) computer system can readily manipulate (thus detect, categorize, identify and act upon), then even non-robotic computer systems could be said to be "sensorimotor" and hence able to "ground" symbols in this narrow domain. 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 (Blondin-Massé et al. 2008). 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 p-zombie, with no one home, feeling feelings, meaning meanings (Harnad 1995). However, it is possible that different interpreters (including different intelligent species of animals) would have different mechanisms for producing meaning in their systems, thus one cannot require that a system different from a human "experiences" meaning in the same way that a human does, and vice versa. Harnad thus points at consciousness as a second property. The problem of discovering the