Quote on testing, checking, and verification I'm in the habit of putting a motivating quote on the top of each of my syllabi. For my introductory programming course, I'd like to focus the class on the theme of testing and verification. Of course, this is also an important theme in non-CS disciplines, as well. 

So I'd like to find the best possible short, positive (not sarcastic or negative), informative quote on the issue of testing and verification. Ideally this would be from someone fairly well-known or influential in the CS field, but that's not necessary. Web searches for this tend to dredge up pages on automatic testing suites and the like, so I'm hoping that someone here knows of a good one.

What is the best motivating quotation on the importance of testing, checking, and verification for freshmen college (STEM) students?
Program testing can be a very effective way to show the presence of
  bugs, but is hopelessly inadequate for showing their absence.
  
  Edsger W. Dijkstra