Our true nature is divine, and that is our real "I", not the mind and the body, with which we are one most of the time. In any perception or enjoyment the "I" gets connected with buddhi (the determinative faculty), buddhi with manas (the deliberative faculty), manas with a particular sense organ, and the sense organ with a sense object.
Both memory and the desire to repeat an action inhere in our samskara, which dwells in our mental storehouse, called chitta. As we discussed earlier, though memory and desire are two different things, they get easily connected in a person who is not alert. When our "I" gets connected with the combination of memory and desire, it is really our will (buddhi) that gets linked to them. When our will gets linked to the desire, harmless images from memory become animated, and connect themselves to the sense organ and the sense object, forcing us to succumb to the desire. The chain of this enjoyment need not always terminate at the gross level; it could stop at the mental level itself.
Desire thus extends from our buddhi through manas to the sense organs, making us blind to our real nature. Says Sri Krishna, "The sense organs, the mind and the intellect (buddhi) are the seat of desire. Through these desire deludes the embodied soul by veiling its wisdom." (Bhagavad Gita, 3.40)
From the above discussion it is clear that it is the will (the dynamic aspect of buddhi) that starts the downward journey by attaching itself to desire. This deliberate attaching of the will to desire is what is called sankalpa, resolve. Parenthetically, it may be said that the customary sankalpa done before a formal worship (puja) has a positive connotation: it is done to consciously connect the wayward will to the act of worship. A famous verse from the Mahabharata ("Shantiparva," 177.25) describes the role of sankalpa in triggering our downfall: "O desire, I know your root. You spring from resolve (sankalpa). I shall not tag my will to you. You will then be destroyed with your root."
The Brihadaranyaka Upanishad (4.4.5) describes the connection between desire, will, and karma: "The Self is identified with desire alone. What it desires, it resolves; as is its resolution, so is its action. And whatever it carries out into action, that it reaps." Sri Shankara comments on this passage: "Desire manifests itself as longing for a particular object, and, if unchecked, it assumes a more definite shape and becomes resolve."
During the initial stage of his struggle with the mind, a spiritual aspirant may not always succeed in detaching his will from desire. As long as it is not a deliberate action on his part, he need not unduly worry about his will getting hooked to the subtle sense organ and the subtle sense object. He only needs to strive with greater effort for purity of mind. That is perhaps what Holy Mother Sri Sarada Devi meant by saying, "In this Kali Yuga mental sin is no sin." The aspirant's sincere struggle with his mind, fortified by prayer and japa, will enable him to gradually gain the upper hand over his unruly mind.