Within.

There is only one way: Go within. Search for the cause, find the impetus that bids you write. Put it to this test: Does it stretch out its roots in the deepest place of your heart? Can you avow that you would die if you were forbidden to write? Above all, in the most silent hour of your night, ask yourself this: Must I write? Dig deep into yourself for a true answer. And if it should ring its assent, if you can confidently meet this serious question with a simple, “I must,” then build your life upon it. It has become your necessity. Your life, in even the most mundane and least significant hour, must become a sign, a testimony to this urge.

- Rainer Maria Rilke