In plain words
The desires that arise in man today are nothing but animal desires. The Upaniṣads describe a true man as one who has no desires of any kind, one who is pure. What makes desires begin in a humanness so pure? The old vāsanās. You must try to get rid of these old vāsanās little by little.
Three kinds of people
In this world men are of three kinds. The first kind of person is like well-carded cotton. Just as cotton turns to ash the moment fire touches it, his heart becomes pure the moment the fire of Jñāna touches it. The second kind of person is like green wood; however close he comes to the fire of Jñāna, his heart does not become pure. And the third kind of person is like dry wood; when he comes close to the fire of Jñāna, his heart becomes pure little by little. By the fruit of the sādhanas of many births, ‘green wood’ turns into ‘dry wood’. Man today is still green wood; he is not dry wood, and cotton he is not at all. For he does not do his sādhanas with determination and with the spirit of sacrifice. Where there is no spirit of sacrifice at all, no spiritual feeling shines in man.
Desires alone foster the demonic nature
Man today not only forgets the help others have given him; he even plots harm against the very one who helped him; sunk in delusion, he takes birth after birth. Karmas alone are the cause, whether births multiply or come to an end. If you want freedom from birth, you must first of all curb your desires. Through attachment to the body, desires go on growing in man. So rākṣasas are not beings with some special form. When man forgets the humanness and the divinity in himself, man himself turns into a rākṣasa. You may have read it in the Rāmāyaṇa: Śūrpaṇakhā longed to marry Rāma and came in the form of a very beautiful woman. But because Rāma would not agree, she turned at once into a rākṣasī.