So there are many volumes of knowledge of this material sky and spiritual sky. In Śrīmad-Bhāgavatam, Second Canto, you will find description of the spiritual sky, what nature it is, what kind of people remain there, what is their feature—everything. Even we get information that in spiritual sky there is aeroplane, spiritual aeroplane in the spiritual sky. And the living entities, they are liberated. They travel on the spiritual sky on that plane, and it is so nice, just like lightning. The description is... They travel just like lightning. So everything is there. This is only imitation. This material sky and everything, whatever you see—all imitation, shadow. It is shadow. Just like in cinematographic picture you see the shadow of the real thing behind, similarly, this is only shadow. And in Śrīmad-Bhāgavatam it is said that janmādy asya yataḥ anvayād itarataś ca artheṣu abhijñaḥ svarāṭ, yatra tri-sargo 'mṛṣā (SB 1.1.1). This material world is a combination of matter. Just like you have seen a nice girl's doll in the showcase of the shopkeeper's exactly just like a nice, beautiful girl. But that is imitation. Imitation. Every sane man knows, "Oh, this is imitation."
So whatever we are seeing here, beautiful, they are all imitation of the real. As the doll is imitation of a beautiful girl, similarly, yasya satyatayā nityāpi satyam eva abhipadyate. Śrīdhara Svāmī says, "Because the spiritual world is real and this unreal manifestation appears to be real, appears to be real, but it is not real, we can understand reality will exist; reality will not vanquish." That is... Reality means eternal. Therefore real pleasure, that is Kṛṣṇa. The material pleasure is temporary, not actual. Therefore those who are after reality, they don't take part in this shadow pleasure. Shadow pleasure, they don't take part. So Lord Kṛṣṇa says that yaḥ sa sarveṣu bhūteṣu naśyatsu na vinaśyati. That is reality. "When everything will be vanquished, that spiritual nature will continue to remain." That remains always.
So our aim is, the human life's aim is, to reach that spiritual sky, but they do not know. Bhāgavata says, na te viduḥ: "They do not know that there is reality." There is reality. Na te viduḥ svārtha-gatiṁ hi viṣṇum (SB 7.5.31). They do not know their self-interest, that this human life is meant for understanding that reality and prepare for being transferred into that real reality, not to remain. The whole Vedic literatures instructs us like that. Tamasi mā jyotir gama: "Don't remain in this darkness." This material world is darkness. We are artificially making it illuminated with electric light and fire and so many things, but the nature is dark. But that nature, that spiritual nature, is not dark. That is full of light. Just like the sun planet, there is no possibility of darkness, similarly, every planet there, they are self-illuminated, so there is no darkness.