Spiritual world is real
SB Canto 7
The spiritual world is real, and the material world is similar to it.
SB 7.15.61, Purport: In material existence there are many regulative principles and formalities. If material existence is temporary or false, this does not mean that the spiritual world, although similar, is also false. That one's material body is false or temporary does not mean that the body of the Supreme Lord is also false or temporary. The spiritual world is real, and the material world is similar to it. For example, in the desert we sometimes find a mirage, but although the water in a mirage is false, this does not mean that there is no water in reality; water exists, but not in the desert. Similarly, nothing real is in this material world, but reality is in the spiritual world. The Lord's form and His abode—Goloka Vṛndāvana in the Vaikuṇṭha planets—are eternal realities.
Bhagavad-gita As It Is Lectures
Because Because the spiritual world is real and this unreal manifestation appears to be real, but it is not real, we can understand reality will exist; reality will not vanquish.
Lecture on BG 8.20-22 -- New York, November 18, 1966: 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.