“A difference which makes no difference is no difference at all.”
As quoted in William James: The Essential Writings (1971), edited by Bruce W. Wilshire, p. xiii
1900s
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William James 246
American philosopher, psychologist, and pragmatist 1842–1910Related quotes
As quoted in Wisdom for the Soul : Five Millennia of Prescriptions for Spiritual Healing (2006) by Larry Chang

1990s, Why "Free Software" is better than "Open Source" (1998)
Context: While free software by any other name would give you the same freedom, it makes a big difference which name we use: different words convey different ideas.
In 1998, some of the people in the free software community began using the term "open source software" instead of "free software" to describe what they do. The term "open source" quickly became associated with a different approach, a different philosophy, different values, and even a different criterion for which licenses are acceptable. The Free Software movement and the Open Source movement are today separate movements with different views and goals, although we can and do work together on some practical projects.
The fundamental difference between the two movements is in their values, their ways of looking at the world. For the Open Source movement, the issue of whether software should be open source is a practical question, not an ethical one. As one person put it, "Open source is a development methodology; free software is a social movement." For the Open Source movement, non-free software is a suboptimal solution. For the Free Software movement, non-free software is a social problem and free software is the solution.

Query 2
Opticks (1704)

“Vermont is a small state which makes an enormous difference.”
Commencement Address at Middlebury College May, 2001 http://web.archive.org/web/20030906163501/http://www.middlebury.edu/offices/pubaff/general_info/addresses/Fred_Rogers_2001.htm
“Kyoya: A single day can make all the difference.”
Source: Ouran High School Host Club, Vol. 2