prakṛti--kāraṇa yaiche ajā-gala-stana
ataeva-therefore; kṛṣṇa-Lord Kṛṣṇa; mūla-original; jagat-kāraṇa-the cause of the cosmic manifestation; prakṛti-material nature; kāraṇa-cause; yaiche-exactly like; ajā-gala-stana-nipples on the neck of a goat.
Therefore Lord Kṛṣṇa is the original cause of the cosmic manifestation. Prakṛti is like the nipples on the neck of a goat, for they cannot give any milk.
The external energy, composed of pradhāna or prakṛti as the ingredient-supplying portion and māyā as the causal portion, is known as māyā-śakti. Inert material nature is not the actual cause of the material manifestation, for Kāraṇārṇavaśāyī, Mahā-Viṣṇu, the plenary expansion of Kṛṣṇa, activates all the ingredients. It is in this way that material nature has the power to supply the ingredients. The example given is that iron has no power to heat or burn, but after coming in contact with fire the iron becomes red-hot and can then diffuse heat and burn other things. Material nature is like iron, for it has no independence to act without the touch of Viṣṇu, who is compared to fire. Lord Viṣṇu activates material nature by the power of His glance, and then the ironlike material nature becomes a material-supplying agent just as iron made red-hot becomes a burning agent. Material nature cannot independently become an agent for supplying the material ingredients. This is more clearly explained by Śrī Kapiladeva, an incarnation of Godhead, in Śrīmad-Bhāgavatam (3.28.40):