Verse 7.18.1

यदा वै मनुतेऽथ विजानाति नामत्वा विजानाति मत्वैव विजानाति मतिस्त्वेव विजिज्ञासितव्येति मतिं भगवो विजिज्ञास इति ॥ ७.१८.१ ॥
॥ इति अष्टादशः खण्डः ॥

yadā vai manute'tha vijānāti nāmatvā vijānāti matvaiva vijānāti matistveva vijijñāsitavyeti matiṃ bhagavo vijijñāsa iti || 7.18.1 ||
|| iti aṣṭādaśaḥ khaṇḍaḥ ||

1. Sanatkumāra said: ‘When a person learns to think well, then he can know deeply. Without thinking well, one cannot know deeply. One knows for certain when one thinks deeply. But one must want to know how to think well.’ Nārada replied, ‘Sir, I want to know how to think well’.

Word-for-word explanation:

Yadā vai manute, when one learns to think well; atha vijānāti, then one can know deeply; amatvā, without applying the mind; na vijānāti, one cannot know deeply; matvā eva vijānāti, a person knows deeply when he thinks deeply; matiḥ tu eva vijijñāsitavya iti, but one must want to know how to think well; bhagavaḥ, sir; matim vijijñāse iti, I want to know what this thinking is. Iti aṣṭādaśaḥ khaṇḍaḥ, here ends the eighteenth section.

Commentary:

How do we know something? We know it by applying our mind—that is, by concentrating our mind on it.

Swami Turiyananda used to say that if you continue reading the Gītā with a concentrated mind, then whenever you read it new meanings will unfold, meanings which you never suspected.

Sanatkumāra says that without applying your mind seriously, you will not understand what you are studying. At Belur Math, Swamiji used to have the monks debate on different issues. One person would say something, and another would contradict him. When two people debate about something, they both get excited and the heat rises. As one of the monks used to say, ‘When there is some heat there will also be some light.’ Debating is like churning milk. As you chum the milk the cream gradually begins to appear. So also, when there is a debate, truth gradually comes to the surface.

Suppose you have a thorn stuck in your foot. What do you do? As Sri Ramakrishna used to say, you take another thorn and use it to remove the first thorn. Then you throw away both. Similarly, you have to use your mind to go beyond the mind. Vedānta says that Truth is within you. But you must first hear about it from a teacher. Then you must reflect on it. Think over it again and again: ‘What does this mean? How can I be one with Brahman?’ You go on questioning, searching for the real meaning of the, words. After that you must deeply meditate on it. Then only the real meaning dawns on you. It comes as if in a flash.

What else is this mind for? If we don’t think, we are as good as dead. Human beings are superior

Śaṅkara gives a wonderful definition of reflection. He says it means having great love for the subject being considered.