Everything is spiritual, but we do not know how to use it; therefore it is material. When you forget Kṛṣṇa, when you forget the proprietorship of Kṛṣṇa upon everything, that is material. Hari-sambandhe. Kṛṣṇa-sambandhe. Nirbandhaḥ kṛṣṇa-sambandhe yuktaṁ vairāgyam ucyate. This is the injunction of the Gosvāmīs, that we have to accept everything as Kṛṣṇa. That is the fact. And we are using everything without utilizing for Kṛṣṇa's purpose; therefore it is material. And in the Bhagavad-gītā it is said, stena eva sa ucyate [Bg. 3.12] = "He is thief. He is thief."
Srila Prabhupada's lecture on Bhagavad-gītā 13.6–7, Sept. 29, 1973, Bombay