“But the amateur who reads poetry from love or curiosity does have at least one advantage over the trained specialist who reads it from professional obligation. Amateurs have not learned to shut off parts of their consciousness to focus on only the appropriate elements of a literary text. They respond to poems in the sloppy fullness of their humanity”

Source: The War of Art: Break Through the Blocks & Win Your Inner Creative Battles

Ego, p. 303, September 17, 1933.

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

Interview with Robert McPhillips http://www.danagioia.net/about/mcphillips.htm (December 1991), published in Verse (Summer 1992)
Interviews

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,

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.

Variant: The man who does not read has no advantage over the man who cannot read.

"Preface"
Between Time and Timbuktu (1972)