Only pure water can purify. The further you wish to take that water, the deeper your wellspring must be.

Those who wish to reach to the very outside and enlighten and purify that place, they must reach deep into the womb of truth.

Likkutei Sichot, vol. 4, p. 1119; Acharon Shel Pesach 5711:10; Torat Menachem 5742, vol. 2, p. 820.