Does dipping only a part of the hand, such as a finger or nail, invalidate purification or count as the full dipping?
Chapter on Siwak and the Sunnah of Wudu
Al-Mughni
Book of Purification
Primary text
Dipping only a part of the hand, even a finger or a nail, is considered equivalent to dipping the entire hand in one ruling, because the prohibition that applies to the whole applies to its parts, similar to ritual impurity (*hadath*) and uncleanness (*najasah*).
Supporting text
A second opinion holds that dipping only a part does not prevent purification (the view of Al-Hasan). This is because the prohibition specifically addressed dipping the entire hand, and what applies to the whole does not necessarily apply to its constituent parts, just as causality does not necessarily transfer from the whole to a part.