Srimad BhagavadGita

As declared in the previous śloka nāsti buddhir-ayuktasya, soul knowledge cannot be completely understood by one who has not gained victory over his senses. This śloka elaborately explains why this is so. Therefore this is not a repetition. Even if some repetition is noticed, it is to be understood that Kṛṣṇa is repeating it to emphasise on the importance of the topic. Kṛṣṇa says to Arjuna, "The mind that is made to follow the senses which crave for the sense objects as they please, certainly pulls his intelligence, towards those sense objects just as headwind forcibly sidetracks a boat sailing in water from its course."

caratāṃ indriyāṇāṃ (The senses which are not in control, due to which they wander freely over the sense objects like sounds, forms, etc, as per their liking) It is not possible for all the senses to be placed on their sense objects and to be dwelling there full time. Hence the word 'caratāṃ' in this context means 'craving of the senses for the sense objects' and not grazing. To support this, we quote indriyāṇ īndriyārtheṣu vartanta iti dhārayan B.G 5.9 where the word 'vartanta' is also interpreted as 'craving'. 

yan manaḥ anuvidhīyate (The mind which is made to follow the wandering senses, by a person) indriyāṇāṃ tu sarveṣām yadyekam kṣaratīndriyam | tenāsya kṣarati prajnā dṛtehe pādādivodakam Manu smṛti 2.99 (Even if one sense is attached to its sense objects, the jīva’s knowledge slips out and gets wasted, just as a single opening is enough for all the water in the container to flow out and get wasted). On the basis of the above Manu smṛti statement and 'indriyāṇāṃ' which is in ṣaṣṭi vibhakti as nirdhāraṇe ṣaṣṭi, one meaning would be, 'Out of all the senses which are wandering around sense objects, whichever sense the mind is made to wander around, that particular sense destroys all the knowledge.’ But this is not acceptable. Reasons are as follows.

1)In the above Manu smṛti statement, the word 'mind' is not used at all, the word indriya is used twice and on the basis of the word 'ekam' in it, it is justified to interpret its meaning as stated above. But that is not the case with this śloka.

2)Moreover, it is the mind which is primarily responsible for the destruction of knowledge, not the senses.

When 'indriyāṇāṃ' is taken to be nirdhāraṇe ṣaṣṭi, the first permutation is, 'An external sense like sense of sight, smell, taste, form, sound is made to follow the mind'. In this case, knowledge will only expand but will not get destroyed. Therefore this is not acceptable here. The second permutation is, 'The mind is made to follow a sense'. In this case knowledge gets destroyed. However this applies equally for all the senses and it is not possible to pinpoint a particular sense alone. Therefore the meaning told by us that, 'The mind which is made to follow the wandering senses destroys the person's intelligence' is appropriate.

tat asya prajñāṃ harati (When a person lets his mind follow the senses, it causes destruction of his intelligence) Such a mind destroys his intelligence which originally desires for the divine soul and makes it desire for the sense objects. An illustration to understand this better is given below.

aṃbhasi nāvam vāyuriva Just as headwind forces a boat travelling in the waters to change its course.