Define the world: the 'set of conditions for the possibility of success.'
Start with a problem or opportunity and brainstorm how to work within it asking UX's 'How might we...?' This is a phase where a lot is literally put on the wall to find what sticks.
For Long Time, Let's See, my team fixated on assisting with impersonal addiction in social media. That took dozens of possible forms from plugins to events to our end-result, a standalone app inspired by dating apps to get people off the app. That wasn't an idle decision- we prototyped and tested rapidly to see if the empathetic and subconscious effects we expected from each solution were actually happening.