So long we are on the platform of mental speculation, we should understand that we are on the material plane, because mind is material. Mind is not spiritual. So mano-gatan. The special word is used here, mano-gatan. Whatever we create in our mind, that is material, all creations. Mind is the leader of the senses. So the activities of the mind - thinking, feeling and willing - are expressed through our senses. And these sensual activities are known as our living condition. Therefore the Lord says, "When one shall be free from mental speculation, then he's to be understood that he is in the perfect stage of spiritual consciousness."
Lecture on Bhagavad-gita 2.55-58, New York, April 15, 1966