When Lord Caitanya began His instructions to Rūpa Gosvāmī, He first told him, "My dear Rūpa, the science of devotional service is just like the great ocean, and it is not possible to show you all its length and breadth. However, I shall try to explain the nature of that ocean by taking just one drop out of it. In this way you can taste it and understand what that ocean of devotional service actually is.
The Lord then explained that within this brahmāṇḍa, or universe, there are innumerable living entities who, according to their own fruitive activities, are transmigrating from one species of life to another and from one planet to another. In this way their encagement in material existence has been continuing since time immemorial. In actuality, these living entities are atomic parts and parcels of the supreme spirit. It is said in the Śrīmad-Bhāgavatam that the length and breadth of the individual soul is approximately 1/10,000th part of the tip of a hair—in other words, it is so small that it is invisible. This is also confirmed in the Śvetāśvatara Upaniṣad. In the Tenth Canto of Śrīmad-Bhāgavatam, one of the four Kumāras, known as Sanandana, gave the following speech upon performing a great sacrifice: "O Supreme Truth! If the living entities were not infinitesimal sparks of the supreme spirit, each minute spark would be all-pervading and would not be controlled by a superior power. But if the living entity is accepted as a minute part and parcel of the Supreme Lord he automatically becomes controlled by a supreme energy or power. The latter is his actual constitutional position, and if he remains in this position he can attain full freedom." (SB 10.87.30) If one mistakenly considers his position to be equal to that of the Supreme Personality of Godhead, he becomes contaminated by the doctrine of nonduality, and his efforts in transcendental life are rendered ineffective.
Lord Caitanya elaborated on these teachings of Śrīmad-Bhāgavatam by pointing out that there are two kinds of living entities—the eternally liberated and the eternally conditioned. The eternally conditioned living entities can be divided into two types—moving and nonmoving. Those entities which cannot move—like trees, for example—remain in one place and are classified as nonmoving entities, and those that move—such as the birds and beasts—are called jaṅgama (moving entities) and are further divided into three categories: those that fly in the sky, those that swim in the water, and those that walk on land. Out of the many millions and trillions of living entities on land, human beings comprise only a small portion. Out of that small number of human beings, most are totally ignorant of spiritual life, are unclean in their habits and have no faith in the existence of the Supreme Personality of Godhead. In short, most human beings live like animals. These can actually be deducted from the number of human beings that comprise human or civilized life. It is very difficult to find a few human beings who believe in the scriptures and the existence of God, or, for that matter, in proper behavior. Those who do believe in the value of these things are known as ārya, a word denoting those who believe in advancing in spiritual life. Out of those who believe in the value of the scriptures and the advancement of human civilization, there are two classes—the righteous and the unrighteous. Those who are righteous generally execute fruitive activities in order to derive some good result for sense gratification. Out of many such persons who engage in righteous activities for sense gratification, only a few come to know about the Absolute Truth. These are called jñānīs, empiric philosophers. Out of many hundreds and thousands of such empiric philosophers, only a handful actually attain liberation. When one is liberated, he theoretically understands that the living entity is not composed of material elements but is spirit soul, distinct from matter. Simply by theoretically understanding this doctrine, one can be called liberated, but actually a mukta, or liberated soul, is he who understands his constitutional position as an eternal servant of the Lord. Such liberated souls engage with faith and devotion in the service of the Lord, and they are called kṛṣṇa-bhaktas, or Kṛṣṇa conscious persons.