Ramana Maharshi said:
As long as there are impressions of objects in the mind, the enquiry “Who am I?” is required. As thoughts arise they should be destroyed then and there in the very place of their origin, through enquiry. If one resorted to contemplation of the Self without a break, until the Self is gained, that alone will do. As long as there are enemies within the fortress, they will continue to sally forth; if they are destroyed as they emerge, the fortress will fall into our hands.