The material body, made up of material ingredients such as earth, water, fire, and air, is mortal. Similarly, because this material universe is an amalgam of earth, water, fire, air, etc., it is also transitory. But the spirit soul (which, incidently, has never been duplicated in the laboratory despite repeated efforts) is imperishable, as is its natural, eternal home—the kingdom of God. The process that takes the eternal soul to his eternal home is called sanātana-dharma, or "the eternal religion."
Empirical, atheistic philosophers like Kapila spent innumerable tedious hours researching the material phenomena of this cosmic creation. Yet it remained beyond the grasp of their limited intelligence to understand that there exists a realm transcendental and far superior to this manifested material world. Finally, when their probing minds failed to sight land in an ocean of speculation, they concluded that the absolute truth is unmanifest.
Compared to other species, human beings are certainly endowed with good intelligence, yet unless they are devotees of the Lord, all their thinking is limited within mundane boundaries. Therefore it is impossible for the mundane mind to approach the transcendence. But instead of surrendering to the Supreme Lord or His representative, the empirical philosophers try to explain away as "unmanifest" that which is beyond their mundane minds. This is known as the logic of the frog in the well.
No matter how big a thinker a tiny living entity may be, all his activities are limited by mundane boundaries, just as a frog in the well can never comprehend that such a thing as an ocean exists outside his little domain. He refuses to acknowledge that a mass of water infinitely bigger than his tiny puddle can at all be possible. Similarly, we are trapped in the dark well of our body and mind. And although we may try hard through yoga or empirical speculation to overcome our limitations, no matter how erudite we are it is impossible to reach beyond the limitations of our self-made well.