Sri Sri Radha Shyamsunder (Sri Vrindavan Dhaam)
The mind of the conditioned soul, on account of its association with the material energy from time immemorial, contains heaps of dirt in the form of desires to lord it over material nature. This dirt is like a mountain, but a mountain can be shattered when hit by a thunderbolt. Meditating on the lotus feet of the Lord acts like a thunderbolt on the mountain of dirt in the mind of the yogi. If a yogi wants to shatter the mountain of dirt in his mind, he should concentrate on the lotus feet of the Lord and not imagine something void or impersonal. Because the dirt has accumulated like a solid mountain, one must meditate on the lotus feet of the Lord for quite a long time. For one who is accustomed to thinking of the lotus feet of the Lord constantly, however, it is a different matter. The devotees are so fixed on the lotus feet of the Lord that they do not think of anything else. Those who practice the yoga system must meditate on the lotus feet of the Lord for a long time after following the regulative principles and thereby controlling the senses.
- Srila Prabhupada (SB 3.28.22 Purport)