Most men are, like animals, driven by the forces of Nature: Whatever desires come, they fulfil them, whatever emotions come they allow them to play, whatever physical wants they have, they try to satisfy. We say then that the activities and feelings of men are controlled by their Prakriti, and mostly by the vital and physical nature.
The body is the instrument of the Prakriti or Nature—it obeys its own nature or it obeys the vital forces of desire, passion, etc. But man has also a mind and, as he develops, he learns to control his vital and physical nature by his reason and by his will.
This control is very partial: for the reason is often deluded by vital desires and the ignorance of the physical and it puts itself on their side and tries to justify by its ideas, reasonings or arguments their mistakes and wrong movements. Even if the reason keeps free and tells the vital or the body, “Do not do this”, yet the vital and the body often follow their own movement in spite of the prohibition—man’s mental will is not strong enough to compel them.
When people do sadhana, there is a higher Nature that works within, the psychic and spiritual, and they have to put their nature under the influence of the psychic being and the higher spiritual self or of the Divine.
Not only the vital and the body but the mind also has to learn the Divine Truth and obey the divine rule. But because of the lower nature and its continued hold on them, they are unable at first and for a long time to prevent their nature from following the old ways—even when they know or are told from within what to do or what not to do.
It is only by persistent sadhana, by getting into the higher spiritual consciousness and spiritual nature that this difficulty can be overcome; but even for the strongest and best sadhaks it takes a long time. But, if you can once get it out of the nature and realise it as a force coming from outside and putting its claws into the vital and physical, it will be easier to get rid of the invader.