Thought is a very elevated level. One who wishes to think only thoughts of holiness and ascend to the world of thought must be completely silent. Even saying something good and noble would spoil the thought. But even when a person is completely silent, there are still many distractions which confuse the mind. He still has to purify his thoughts.
Sexual fantasies are the prime source of all impurity.
They are as strong a source of pollution as a dead body. Charity gives some protection against these fantasies, but one should never rely on this and permit oneself to talk freely with women. One should say only what is essential. There are certain fantasies which derive their strength from a particular husk which is extraordinarily tough. It is very difficult to escape from them.