In 1948, W. Grey Walter built two simple robots that could sense light and touch.[1] Much complex behavior emerged. That's where this idea started.
Watch the video.[2] It's very impressive for 1948 and tube technology. If you've seen an original Roomba, you've seen a similar level of intelligence. Rod Brooks went down this road. But it didn't go very far. There are severe limits to purely reactive robotics. It doesn't work well for anything that moves fast enough that inertia matters.
Fun historical note: the linked newsreel film was known about but unseen for a long time, then found by Owen Holland in the BBC Bristol archive around 1999. Either Owen or I sent it to Rod, to great enjoyment.
Film includes operator-moves-trashcan-with-foot, a staple of mobile robot clips ever since.
Walter himself mentioned the perceived complexity of a simple system to the observer, stating it "might be accepted as evidence of some degree of self-awareness."