Her only test of probably truth is what works best inthe wya of leading us, what fits every part of life best and comines with the collectivity of experience’s demands, nothing being omitted.

William James (1842-1910) American psychologist and philosopher

Pragmatism: A New Name for Some Old Ways of Thinking, Lecture 2, “What Pragmatism Means” (1907)

Full text. "Her" is "Pragmatism." Often paraphrased as "Truth is what works."