Ad fontes is a Latin expression which means "[back] to the sources" (lit. "to the fountains"). The phrase epitomizes the renewed study of Greek and Latin classics in Renaissance humanism. Similarly, the Protestant Reformation called for renewed attention 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.
This phrase is related to Ab initio, which means "from the beginning." Whereas ab initio implies a flow of thought from first principles to the situation at hand, ad fontes is a retrogression, a movement back towards an origin, which ideally would be clearer than the present situation.
The phrase ad fontes occurs in the Latin Vulgate version of Psalm 42:
quemadmodum desiderat cervus ad fontes aquarum ita desiderat anima mea ad te Deus.
(In the same way that the stag is drawn unto the sources of water, so is my soul drawn unto you, God.)
According to Hans-Georg Gadamer, there is evidence provided by E. Lledo that Spanish humanists drew the expression from this source.
Erasmus of Rotterdam used the phrase in his De ratione studii ac legendi interpretandique auctores:
Sed in primis ad fontes ipsos properandum, id est graecos et antiquos.
(Above all, one must hasten to the sources themselves, that is, to the Greeks and ancients.)