ad fontes

Ad (“to”) + fontēs, accusative plural form of fōns (“fountain”, “spring”, “source”) = literally, “to the springs or sources”; probably drawn from Psalm XLII, verse 1^[1] of the Latin Vulgate by Spanish humanists.^[2]

• (Classical Latin) IPA^(key): /ad ˈfon.teːs/, [äd̪ ˈfɔn̪t̪eːs̠]
• (modern Italianate Ecclesiastical) IPA^(key): /ad ˈfon.tes/, [äd̪ ˈfɔn̪t̪es]

ad fontēs

1. (idiomatic) Go to the sources: An expression emphasizing the importance of conducting fundamental research and of consulting primary sources.

• A motto of Renaissance humanists. Similarly, the Protestant Reformation called for the return to the Bible as the primary source of Christian faith. The idea in both cases was that sound knowledge depends on the earliest and most fundamental sources.

• ab initio (“from the beginning”)