Purification of the soul aims at clearing one’s heart of all kinds of negative sentiments, There is a very interesting story, which illustrates this point –
A milkman, deeply influenced by Gautama Buddha, requested Buddha to visit him and share with him nuggets of wisdom. In lieu of this, the milkman offered to present the seer with milk. Buddha agreed.
In the evening, when Buddha set out to visit the milkman, he took with him a container in which he intentionally put some mud. The milkman took the container but just as he was about to pour milk into it, realized that the container had some impurities in it.
The milkman washed the container clean. He then poured milk into it and gave it to Buddha. Picking up the container, Buddha got up to leave. Surprised, the milkman asked Buddha why he was leaving before imparting any wisdom. Buddha replied that he just had. Confused, the milkman urged Buddha to explain what he meant. Buddha told the milkman: “The mind is like the container. Thoughts that preoccupy us are like the impurities in the container. In order to gain wisdom, you must purify your mind by making it free of impure thoughts. Only when your mind is uncluttered, it will be able to receive wisdom.”
In a nutshell, what Gautama Buddha was trying to teach the milkman was that the mind needs to be prepared first – by cleansing it of impurities – so that it is fully prepared to receive wisdom.