Diane, you're half right in your recollection.
The plaque that hangs on Bascom Hall symbolizes the university's part in the struggle for academic freedom and reads:
"Whatever may be the limitations which trammel inquiry elsewhere, we believe that the great state University of Wisconsin should ever encourage that continued and fearless sifting and winnowing by which alone the truth can be found."
Sifting and winnowing had special meaning in Wisconsin, an agricultural state. Farmers sift grain to separate the fine from the coarse, and winnow to separate the grain from the chaff. Today, this statement remains the credo of the university.
The wall and inscription you referred to in your question also exists, but they are part of South Hall (on the upper campus between the Law Building and Bascom Hall). The full text reads, "Ye shall know the truth, and the truth shall make you free. Class of 1955." The quote is from the Bible (John 8:32) and was approved for inscription on South Hall by the Board of Regents on June 14, 1955.