Levels Hypothesis
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The so-called Levels Hypothesis claims that some devices can be described at a variety of levels...it will suffice to distinguish three such levels. They are:
Computers and humans are, according to this hypothesis, both devices which can appropriately described at each of these levels. (A lump of coal, a red apple, etc. are examples of things that aren't appropriately described at these various levels...only rather special things can be described at all of these levels.) What is a level? Intuitively, it is a way of describing things. When we speak of our brain, its neurons, their organization, and the like we are describing things at the Physical Level. In addition to the way we carve up and name the "stuff" at this level, there are also laws, in this case physical laws, that describe the way in which the "stuff" behaves. Computers can be described at the hardware level, chips, transistors, bus, etc. And, the physical laws of electricity hold in describing this level. Thus, a level represents a kind of commitment to the existence and lawful behavior of certain kinds of entities. The Symbolic or Computational Level is a level at which we describe symbols, expressions composed from symbols, processes that map from expressions to expressions, and the like. The Turing Machine was, of course, a description at this level. Now, the idea is that there are usually many ways to physically realize or instantiate a computational device. Nonetheless, the computational description and laws describe the device quite independently of its physical instantiation. The Semantic or Knowledge Level is the level at which we describe the notion of a rational agent. One sense of this is that a device is rational to the extent that it uses its knowledge to attempt to satisfy its goals. Another, broader and more technical sense, is that a computational device realizes some semantics if and only if it generates outputs that are derivable from some well-specified semantics of the domain. |
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| In order to
simplify the discussion, let use turn to the game of Tic Tac
Toe and try to use it to illustrate some of these distinctions.
An illustration of the idea that there are many ways to physically
realize something that is described computationally can be viewed
by clicking on Tic Tac Toe Example. |
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The three different physical settings within which Tic Tac Toe could be played are clearly quite different. However, all that matters about the physical setting is that it provide a way in which to represent the computational ideas of a move; of the entities controlled by each player at each point in the game; and finally, a relation that holds over 8 subsets of size 3 of the entities. Viewing the animation of a game in these three settings illustrates that the physical similarity between the traces of a game can be fairly substantial to non-existent. Now, reflect for a moment on the various possible sequences of events that are possible at each level. At the physical level, any physically realizable sequence of moves is possible. For example, as depicted in the figure to the right, 3 X moves could immediately be made across the top row. This sequence violates the rules of Tic Tac Toe, but certainly no physical law prevents it. Thus, if we find that only "legal" Tic Tac Toe sequences are observed, then either we must assume that these regularities arise from some other level or there is some Physical "Tic Tac Toe" Law that we have yet to discover. |
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| Similarly, at the level of the game the set of legal move sequences includes moves that we would view as quite "irrational". For example, we find it strange if a player's next move could win the game, but the player makes some other non-winning move. The figure to the right provides an example of this "irrational" case. The "semantics" of a game is to try to achieve the goal of winning. Sequences of moves can violate this "semantics" but still constitute a syntactically correct game. Thus, we have the set of possible Tic Tac Toe games and the set of competitive Tic Tac Toe games. And, this latter set is a subset of the former. |  |
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| There seems to be only a small set of devices which can also be viewed at this semantic level where we consider not only what the device can do but what it "ought" to do if it is considered to be "rational." These remarks point to the potential relevance of normative ideas and models to the study of reasoning and intelligence. The levels hypothesis claims that the mind can be viewed from these three related but distinct perspectives. Hopefully, the Tic Tac Toe example will help you to remember these differing perspectives on the mind. | ||