I) Prevention (Samvara)
The impurities enter the mind through the senses. They enter when we focus our attention on the sensory objects. To prevent this we withdraw our attention from sensory objects, which is to stop reflecting on the pleasantness or the unpleasantness of the object. This is called guarding the senses. Let us examine the words of the Buddha in translation, as it is found in the Sutta.
Herein, a meditator, seeing an image with the eye, is not preoccupied with what is seen or with what is associated with it, so that attraction, repulsion and evil, unprofitable states of mind may not flow into the mind, as it would if one dwelled with this eye-faculty uncontrolled, and did not apply oneself to such control, or set guard over the eye-faculty.
Hearing a sound with the ear, or smelling an odour with the nose, or tasting a savour with the tongue, or contacting tangibles with the body, or cognising mental states with the mind, one is not preoccupied with their general features, or by their associations so that attraction, repulsion and evil,
unprofitable states of mind may not flow upon one, as it would, if one dwelled with ones ear, nose, tongue, body, or mind uncontrolled and did not apply oneself to such control, or set guard over these faculties.
This, in fact, is a withdrawal of attention from what ever is perceived through the senses. It is closing our mind to external objects perceived. This way we are being free of disturbances within due to environmental influences. In other words, we are preventing the mind from being polluted by forces coming from outside. This way we also stop reacting to stimulation of the senses by the environment.