“One finds limits by pushing them” is a popular quotation that is usually credited to American political scientist and economist Herbert Simon (1916-2001). However, the quotation has not been found in Simon’s voluminous writings and Simon has received credit only since about 2010.
 
“One finds limits by pushing them” appears to be first cited in the book Foundations of Cognitive Science (1989) by Michael I. Posner.
 
 
Wikipedia: Herbert A. Simon
Herbert Alexander Simon (June 15, 1916 – February 9, 2001) was an American political scientist, economist, sociologist, and psychologist, and professor—most notably at Carnegie Mellon University—whose research ranged across the fields of cognitive psychology, cognitive science, computer science, public administration, economics, management, philosophy of science, sociology, and political science. With almost a thousand very highly cited publications, he is one of the most influential social scientists of the 20th century.
 
Google Books
Studies in Machiavellianism
By Richard Christie and Florence L. Geis
New York, NY: Academic Press
1970
Pg. 135:
He may test limits by pushing them, but know how not to break them.
   
Google Books
Foundations of Cognitive Science
By Michael I. Posner
Cambridge, MA: MIT Press
1989
Pg. 37:
One finds limits by pushing them.
   
Google Groups: mailing.freebsd.questions
Søren Neigaard
10/2/01
(...)
“One finds limits by pushing them.”
   
Mailing List Archive: Linux Virtual Server: Users
neigaard at e-box
Feb 23, 2002, 3:45 AM
(...)
“One finds limits by pushing them.”
   
Harald Felgner & the Red Fez
Business Quotes: Efficiency and Effectiveness, Innovation, and Change
By Harald Felgner on July 22, 2010 5:17 AM
(...)
16. “One finds limits by pushing them.” Herbert Simon
   
Getting Even Faster
Sunday, January 8, 2012
“One Finds Limits By Pushing Them.”
“One finds limits by pushing them.”
-Herbert Simon
 
Footsteps
Thought for This Day 9.14.12
“One finds limits by pushing them.”
-Herbert Simon