Bhaktivinoda thakura in his Chaitanya Sikshamrta chapter 3 says:
“The cultivation of God can be found in karma marga but it is a secondary part of morality. It is not devotion, because moral conduct is the master and worship of God is but a servant of morality. On the path of jnana, there is cultivation of pure knowledge of Brahman. There, knowledge is the master and worship of God is the servant. That also is not bhakti. In bhakti, God is the only object of cultivation “( chaitanya sikshamrta ).
In that same chapter ,bhaktivinoda thakura gives a good example of why karma is the cause of material desires
“Marriage is an action; its goal is children, who should perform action, in the form of offering rites when the parents die. When this goal is achieved and turns to action of offering the pindas, the goal is to satisfy the pitras. The satisfaction of the pitras leads to the satisfaction of the children. The children then perform more actions, which give more results, which leads to more actions”
Thus in this way chain of action and reactions continues and thus such activities (under karma khanda) becomes the cause of material desires.
Thus as far as karma and jnana are not conducive to the devotional service ,it is taken to be material ,and not spiritual .
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