Although all three potencies - namely internal, external and marginal - are essentially one in the ultimate issue, they are different in action, like electric energy, which can produce both cold and heat under different conditions
"One who knows the real feature of Śrī Kṛṣṇa and His three different energies cannot remain ignorant about Him."
Śrīla Jīva Gosvāmī states in his Bhagavat-sandarbha (16) that by His potencies, which act in natural sequences beyond the scope of the speculative human mind, the Supreme Transcendence, the summum bonum, eternally and simultaneously exists in four transcendental features: His personality, His impersonal effulgence, particles of His potency (the living beings), and the principal cause of all causes. The Supreme Whole is compared to the sun, which also exists in four features, namely the personality of the sun-god, the glare of his glowing sphere, the sun rays inside the sun planet, and the sun's reflections in many other objects. The ambition to corroborate the existence of the transcendental Absolute Truth by limited conjectural endeavors cannot be fulfilled, because He is beyond the scope of our limited speculative minds. In an honest search for truth, we must admit that His powers are inconceivable to our tiny brains. The exploration of space has demanded the work of the greatest scientists of the world, yet there are countless problems regarding even fundamental knowledge of the material creation that bewilder scientists who confront them. Such material knowledge is far removed from the spiritual nature, and therefore the acts and arrangements of the Absolute Truth are, beyond all doubts, inconceivable.
The primary potencies of the Absolute Truth are mentioned to be three: internal, external and marginal. By the acts of His internal potency, the Personality of Godhead in His original form exhibits the spiritual cosmic manifestations known as the Vaikuṇṭhalokas, which exist eternally, even after the destruction of the material cosmic manifestation. By His marginal potency the Lord expands Himself as living beings who are part of Him, just as the sun distributes its rays in all directions. By His external potency the Lord manifests the material creation, just as the sun with its rays creates fog. The material creation is but a perverse reflection of the eternal Vaikuṇṭha nature.
These three energies of the Absolute Truth are also described in the Viṣṇu Purāṇa, where it is said that the living being is equal in quality to the internal potency, whereas the external potency is indirectly controlled by the chief cause of all causes. Māyā, the illusory energy, misleads a living being as fog misleads a pedestrian by blocking off the light of the sun. Although the potency of māyā is inferior in quality to the marginal potency, which consists of the living beings, who are part and parcel of the Lord, it nevertheless has the power to control the living beings, just as fog can block the actions of a certain portion of the sun's rays although it cannot cover the sun. The living beings covered by the illusory energy evolve in different species of life, with bodies ranging from that of an insignificant ant to that of Brahmā, the constructor of the cosmos. The pradhāna, the chief cause of all causes in the impersonal vision, is none other than the Supreme Lord, whom one can see face to face in the internal potency. He takes the material all-pervasive form by His inconceivable power. Although all three potencies—namely internal, external and marginal—are essentially one in the ultimate issue, they are different in action, like electric energy, which can produce both cold and heat under different conditions. The external and marginal potencies are so called under various conditions, but in the original, internal potencies there are no such conditions, nor is it possible for the conditions of the external potency to exist in the marginal, or vice versa. One who is able to understand the intricacies of all these energies of the Supreme Lord can no longer remain an empiric impersonalist under the influence of a poor fund of knowledge.