Many people will shake their heads wistfully and say that they have not found such a thing. Many will not be sure such a thing exists. We are never sure it exists: except in the rare and compelling moments when we feel it.
We do feel it - it's not possible to prove that everybody feels it, but most people feel it - from time to time, or maybe only in a moment that passes too quickly. Flow is more than being so caught up in a thought that everything else fades into the background. Flow is being caught up in a thought that means - a thought that has purpose. We feel it, and we think it, and then we step into a different flow: the flow of change.
You were born for both these flows: the flow of presence, when you are aware of your purpose and the flow of power, when acting out your purpose subjects it to change.