Material knowledge, any subject matter you can take, but that is temporary. Just like this body is temporary, similarly, any material knowledge you acquire, either you become a chemist or physicist or a medical man or an engineer, whatever you may acquire knowledge, all this knowledge will finish as soon as this body is finished. You forget. Death means forgetfulness.
Because the spirit does not die—eternal—so spiritual knowledge continues. If you develop spiritual knowledge . . . suppose cent percent spiritual knowledge you acquire in this body. Then that will continue with you. Even after destruction of this body that spiritual knowledge will continue with you, and when you get next body, you begin . . . you finished your ten percent; you will begin again from eleven percent. That knowledge will not be lost. That is the law of nature. Spiritual knowledge . . . in the Bhagavad-gītā we have studied already, svalpam apy asya dharmasya trāyate mahato bhayāt (BG 2.40).
If you cultivate this spiritual knowledge even one percent, two percent, that can render you greatest service, because it will continue. Once spiritual knowledge begun, it will not be stopped. The best thing is to finish it cent percent in this life, because this human form of life is meant for cultivating spiritual knowledge. It is not meant for material enjoyment.
Material enjoyment means eating, sleeping, and defending and mating. These four principles, they are called material enjoyment. Just like we see advertisement. The other day I was seeing the New York Times magazine. So all advertisements are based on mating. That's all. So because mating is most attractive, therefore the shopkeepers, they advertise their dress, putting before one very nice girl. Because our attraction is for mating, so as soon as we see a nice girl, our attention is diverted immediately. That is the psychology.
So these are all material enjoyments: eating, sleeping, defending and mating. And spiritual enjoyment is just opposite. There is no sense enjoyment. There is self-realization, or purifying the senses. Spiritual enjoyment means purifying the senses. Sarvopādhi-vinirmuktaṁ tat-paratvena nirmalam (CC Madhya 19.170). It is a purificatory process. And as soon as you purify your senses, then you become in full Kṛṣṇa consciousness and become eligible for being transferred to the spiritual world. So this human form of life is specially meant for cultivation of spiritual knowledge. Because in any other form of life . . . there are 8,400,000's of species of life, and the best developed consciousness form of life is this human form of life, especially civilized men.