it would be possible to produce illusions by operatin g at any of these levels. Most difficult of all would be to construct a "picture-image"'by arousin!, from inside the brain, the million lowest-level sensor-agents involved in real-wo*rld vision. Perf,aps the simplest way oi all would be to force iust the highest-level vision-agents into whichever states would result from seeing a certain scene: this would only require some suitable KJines. Let's call this a simulus- a reproduction of only the higher-level effects of a stimulus