It is possible to purify something which has been contaminated with filth but it is impossible to purify something which is itself filth. It is impossible to purify excrement, even if you washed it twenty times. The filth would in fact increase.
Likewise, through an intention, an act that is merely permissible can become something that is recommended or it can become something that is forbidden. Eating is praiseworthy if it is done with the intention to gain strength to worship Allah. However, an intention has no effect on an act that is forbidden. Stealing someone’s money with the intention of giving it to the poor does not make stealing permissible.
People often get their priorities wrong. They are not concerned about whether their food and drink are lawful but when it comes to impurities they are over-cautious. They might wash something twenty times out of fear of it being contaminated by filth. The Companions and the pious people of the Ummah, on the other hand, were extremely concerned about the lawfulness of their income and not overly concerned about impurities. Habib `Alawi bin Shihab said: “If only people were scrupulous about their food and drink and not about purification. Any deficiencies in purification are much more likely to be overlooked.”
Imam al-Ghazali said that when it comes to purification people think things that are highly unlikely (ie that something is impure) are a real possibility. But when it comes to money and food they think things which are a real possibility (ie that it is from an unlawful source) are highly unlikely. This is because they are not well-grounded in taqwa and scrupulousness.
Extracts from Sayyidi Habib Umar bin Hafiz’s classes on the rulings and etiquettes of purification from Habib Abdullah bin Husayn Balfaqih’s “Kifayat al-Raghib” in Shafi fiqh.