[benilov]

It's possible to start agile projects without requirements but with loads of user research and prototyping instead.

[Jtsummers]

From the linked post:

  Before you start writing software, have at least one
  clear objective. You might call this a requirement, a
  user story, or a specification. Give the developers a
  target to shoot at. Otherwise, you will be swimming
  upstream.

Prototypes and research give you a target, even though they aren't "formal" (by some definition) requirements.

Even producing the prototypes meant you probably had some objectives unless someone just typed something out one day entirely on a whim.