SHRDLU (1968) › The Blocks World: A Deliberately Narrow Test

Winograd chose the blocks world to make a difficult problem tractable with 1960s computing resources and AI techniques. The domain still required language interpretation and spatial reasoning, but its objects and permitted actions could be enumerated in advance.

In this simplified domain, the vocabulary was finite and unambiguous. Each object had clear, well-defined properties: color, shape, size, and position. There was no confusion about what "the red block" meant, no metaphorical language to interpret, no ambiguity about whether "pyramid" referred to an Egyptian monument or a geometric shape. This clarity allowed the system to focus on the structure of language itself rather than struggling with lexical ambiguity.

The semantics of actions were equally clear and well-defined. Commands like "pick up," "put down," and "stack" had precise, executable meanings. Each action corresponded to a specific sequence of operations the robot arm could perform, and the effects of these actions on the world state were completely predictable. This meant that SHRDLU could verify its understanding through action: if it correctly understood a command, executing that command would produce the expected result in the simulated world.

The blocks world also required spatial reasoning. To understand commands and answer questions, SHRDLU had to interpret relationships such as "on top of," "under," "beside," "taller than," and "inside." Doing so required a structured representation of the three-dimensional arrangement and an account of how actions changed it.

By avoiding open-domain conversation, Winograd could concentrate on sentence structure, reference, and the connection between language and a represented world. The constraint was central to both SHRDLU's success and its later limitations.