Modification of mind is known as pariṇāma. When milk is changed into curd, it is also pariṇāma. Even so the mind gets modified into a vrtti by assuming the form of the object it perceives. Knowledge of perception is a kind of transformation (pariṇāma) of the mind.
Some people sleep with their eyes open. The object is there. The centre of vision is there and yet they cannot see the object. Why? Because the mind is not there. Therefore three things are necessary for perception of an object, viz., the physical instrument, the external fleshy eyes which act as windows to the soul; the centre of the vision in the brain and the mind. It is the mind that really sees. It is the commander of the five senses. The senses carry the vibrations from outside to the mind. They are avenues of sense-knowledge.
According to Sankhya philosophy, it is the puruṣa or soul that is the real perceiver of an object. The sense transmits the sense-vibrations to the mind, and the mind in its turn presents the vibrations to the prime minister of the soul, the buddhi or intellect, the discriminative faculty, which is very close to the soul. Now, reaction comes in. With the reaction of the buddhi egoism manifests side by side. Then the whole thing is presented to puruṣa who really perceives all objects.
If the physical instrument, eye, is not in a good working order owing to cataract or any other disease, you cannot have perception of an object. The eye may be all right, but if the centre of vision that is situated in the occipital lobe of the brain at the back of the skull does not function properly owing to the diseases of this particular lobe of the brain, in that case also you can have hardly any perception of any object. The eye and the centre of the vision may be in sound condition, but if the mind is not linked with the centre and the external eye, you can hardly have perception of any object. Sometimes a man says: “I did not hear. I did not see. My mind was elsewhere.” You will find such instances in your daily life.