In the donkey case, purity and purifying capacity are distinct concepts. The uncertainty is whether water mixed with its saliva remains legally effective for wudu.
If it were certainly impure, wudu would not be prescribed; if certainly purifying, tayammum would be unnecessary. Combining the two responds to the uncertainty.
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Su’r: Categories, Purity, and Juristic Differences
In the donkey case, purity and purifying capacity are distinct concepts. The uncertainty is whether water mixed with its saliva remains legally effective for wudu.
If it were certainly impure, wudu would not be prescribed; if certainly purifying, tayammum would be unnecessary. Combining the two responds to the uncertainty.