A fish, when it is put into the land, is unhappy, or death. Similarly, if you, the creature of the land, are put into the water, you are unhappy. And death. So because we belong to the spiritual nature… As it is explained by Krsna, this material nature is apara, apara. Apara means inferior, not fit for us. Therefore we are unhappy. So long we shall remain in the material nature, we must be unhappy.
From Srila Prabhupada’s lecture on Bhagavad-gita 4.10 — 2 August, 1974