From the  Q&A section (found July 2010) http://www.philip-pullman.com/q_a.asp?offset=60 
Pullman's website 
Context: If you're going to make a living at this business - more importantly, if you're going to write anything that will last - you have to realise that a lot of the time, you're going to be writing without inspiration. The trick is to write just as well without it as with. Of course, you write less readily and fluently without it; but the interesting thing is to look at the private journals and letters of great writers and see how much of the time they just had to do without inspiration. Conrad, for example, groaned at the desperate emptiness of the pages he faced; and yet he managed to cover them. Amateurs think that if they were inspired all the time, they could be professionals. Professional know that if they relied on inspiration, they'd be amateurs.
                                    
“This script, it seems to me, is the work of professionals who yearned to be as charming as inspired amateurs can sometimes be.”
            "Preface" 
Between Time and Timbuktu (1972)
        
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Kurt Vonnegut 318
American writer 1922–2007Related quotes
“An old maxim says that a professional writer is an amateur who didn't quit”
                                        
                                        A Gift of Wings (1974) 
Source: https://books.google.de/books?id=InUpHgnif58C&pg=PA9&lpg=PA9&dq=An+old+maxim+says+that+a+professional+writer+is+an+amateur+who+didn%27t+quit&source=bl&ots=RpDceaKvsx&sig=ACfU3U0n2qLBUs3E_5CDTfLDvLPmk3tB7A&hl=de&sa=X&ved=2ahUKEwiB16L5_5jyAhWlgf0HHeQyD2oQ6AF6BAgREAM#v=onepage&q=An%20old%20maxim%20says%20that%20a%20professional%20writer%20is%20an%20amateur%20who%20didn't%20quit&f=false A Gift of Wings
                                    
“Amateurs look for inspiration; the rest of us just get up and go to work.”
“Amateurs sit and wait for inspiration, the rest of us just get up and go to work.”
Source: On Writing: A Memoir of the Craft
                                        
                                        Lecture notes ms. (c. 1935); as quoted in: Curt D. Meine, Richard L. Knight (1999) The Essential Aldo Leopold: Quotations and Commentaries. p. 162. 
1930s
                                    
Source: The compleat violinist: thoughts, exercises, reflections of an itinerant violinist http://books.google.co.in/books?id=qC0xAQAAIAAJ, Summit Books, 1 April 1986, p. 79
Source: The War of Art: Break Through the Blocks & Win Your Inner Creative Battles
“Only amateurs attack machines; professionals target people.”
Semantic Attacks: The Third Wave of Network Attacks, 2000-10-15, Schneier, Bruce, Schneier on Security blog, 2010-08-31 http://www.schneier.com/crypto-gram-0010.html#1,