
“The mind is like an iceberg.”
According to this Google Search result https://www.google.com/search?lr=lang_en&tbs=cdr%3A1%2Ccd_min%3A1800%2Ccd_max%3A1920%2Clr%3Alang_1en&tbm=bks&ei=RxfhW-PSCdfbhwOOwYrYBQ&q=%22the+mind+is+like+an+iceberg%22&oq=%22the+mind+is+like+an+iceberg%22&gs_l=psy-ab.3...364980.365440.0.366452.4.4.0.0.0.0.108.108.0j1.1.0....0...1c.1.64.psy-ab..3.0.0....0.cttgYpsxz1o, the earliest reference on this quote belongs to Hon. B. G. Northrop in 1884. But the person who popularized it might be G. Stanley Hall https://en.wikipedia.org/wiki/G._Stanley_Hall.
Misattributed