Google AI:

The phrase “ever not quite” is a famous expression by the American philosopher and psychologist William James that captures his belief that human knowledge and reality are never completely finished or perfectly understood. Instead of a neat, closed world where everything is certain, James argued that the universe is always open to growth, change, and new perspectives.

What the Phrase Means

• The Universe is Incomplete: James did not believe in a “ready-made” universe. He viewed reality as a work in progress where there is always something “not yet considered” or escaping our total grasp.  
• Knowledge has Limits: No matter how smart we are or how much scientific data we gather, human understanding will remain “ever not quite” perfect. There is always a margin for error, growth, and revision. 
• Room for Hope: Because the world is not set in stone, James believed our individual choices, actions, and beliefs truly matter and can help shape the future.

There is a very old saying about philosophy, brought up to date by James’s friend, the mathematician-philosopher Alfred North Whitehead, that philosophy begins in wonder, and after philosophy runs its course, the wonder remains. James’s early reading of Blood drew him to a similar conclusion. Philosophy could only go so far in explaining the mystery of life and reality—and this is ultimately for the best. In the end, the response to the universe in its complexity may not be a white-knuckled attempt to grasp it fully, but a willingness to experience its utter perplexity.

Be Not Afraid of Life: In the Words of William James
William James
John Kaag (Editor), Jonathan van Belle (Editor)
Above quote from their introduction to the essay, “A Pluralistic Mystic”.