Knowledge, however, demands more responsibility of a writer and makes writing more difficult. Trying to write something of permanent value is a full-time job even though only a few hours a day are spent on the actual writing. A writer can be compared to a well. There are as many kinds of wells as there are writers. The important thing is to have good water in the well and it is better to take a regular amount out than to pump the well dry and wait for it to refill. I see I am getting away from the question, but the question was not very interesting.
The well is where your “juice” is. Nobody knows what it is made of, least of all yourself. What you know is if you have it, or you have to wait for it to come back.
An excerpt from George Plimpton’s interview with Earnest Hemingway, The Art of Fiction No. 21, The Paris Review