That chair sure does look comfy, doesn't it? It's a good thing, too, because it's the boy's thinking chair. And when you're doing as much thinking as this kid, you'll need one comfy, cozy chair to sit down in.
The chair is only referenced twice in the poem, but it appears over and over again in the illustrations. The boy starts out sitting on the chair, and he often brings the chair with him whenever he follows a Hunch. In the case of the Up Hunch, he carries the chair all the way up a set of steep, mountainous steps (22.3). The chair also appears in the illustration at the book's end when the boy triumphantly rises from it to claim his luncheon prize.
So the question is, why is the chair included so prominently in Seuss's drawings? Of course, there could be as many different answers as there are readers who are perplexed by the chair's importance. For our money, the chair is Seuss's way of slyly suggesting the whole venture takes places inside the kid's head.
The thinking chair's presence in the oddest of places is the boy's link to his reality where he sits idly in the chair—just think, think, thinking. For us, this explains why the chair multiplies with the boy in during the story's finale. To be fair, though, the opposite reading of the chair's presence is possible. You could argue that the boy bringing it up the hill suggests that he physically left his house to follow the Up Hunch.
What do you think? Does the thinking chair support the Hunches being nothing more than figments of the boy's imagination? Or does it suggest how real his surreal adventure is? Or something else entirely?
If nothing else, it's super suitable that a book such as this would only lead us to Hunches rather than answers.