The idea is that spirit-soul is eternal and the material body is temporary, or for the sake of sense gratification. The mental plane is also material. People being forced to stay on the material platform, they are planning for temporary happiness, and being frustrated, they are jumping from one platform to another which are all temporary. But, fortunately, if one becomes situated on the platform of Krishna Consciousness, then his jumping over material platforms becomes ceased.
Letter to Professor Stillson Judah, 11 June, 1975