SHRDLU (1968) › Capabilities in the Conversation
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The short exchange exercises several language-processing capabilities in sequence. The important details lie in how SHRDLU responds to ambiguity, a relative clause, and a reference to its current world state.
When the human asks SHRDLU to "Grasp the pyramid," more than one pyramid may satisfy the description. The system detects that ambiguity and asks, "I don't understand which pyramid you mean." This is reference resolution with an explicit clarification step: SHRDLU identifies that the noun phrase does not select a unique object.
The command "Find a block which is taller than the one you are holding and put it into the box" combines memory and comparison with reference resolution and planning. SHRDLU must represent the object it holds, compare that object's height with other blocks, identify a qualifying block, and plan the actions needed to place it in the box. The example shows how a nested relative clause can be translated into a sequence of operations.
SHRDLU also makes its handling of the pronoun "it" explicit. Its response, "By 'it', I assume you mean the block which is taller than the one I am holding," states the selected antecedent before acting. The user therefore has an opportunity to correct the interpretation.
The final question, "What does the box contain?" demonstrates yet another capability: the ability to query the current world state and report it in natural language. SHRDLU must identify the box (using reference resolution again), examine which objects are located inside it, and generate a grammatically correct response that enumerates those objects: "The blue pyramid and the blue block." This requires not just perception and knowledge representation but also natural language generation, the ability to produce appropriate linguistic responses to questions.
Together, these capabilities illustrated a model-based approach to language understanding. SHRDLU built representations, reasoned over them, and connected utterances to its model of the world.