Cow's milk is particularly essential for developing the finer tissues of the human brain so that one can understand the intricacies of transcendental knowledge.

Srimad-Bhagavatam

SB Canto 1

The protection of cows maintains the most miraculous form of food, i.e., milk for maintaining the finer tissues of the brain for understanding higher aims of life.

SB 1.8.5, Purport: 

In the glorious days, or before the advent of the age of Kali, the brāhmaṇas, the cows, the women, the children and the old men were properly given protection.

1. The protection of the brāhmaṇas maintains the institution of varṇa and āśrama, the most scientific culture for attainment of spiritual life.

2. The protection of cows maintains the most miraculous form of food, i.e., milk for maintaining the finer tissues of the brain for understanding higher aims of life.

3. The protection of women maintains the chastity of society, by which we can get a good generation for peace, tranquillity and progress of life.

4. The protection of children gives the human form of life its best chance to prepare the way of liberty from material bondage. Such protection of children begins from the very day of begetting a child by the purificatory process of garbhādhāna-saṁskāra, the beginning of pure life.

5. The protection of the old men gives them a chance to prepare themselves for better life after death.

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