As for the difference between mental speculation and philosophical speculation, we take it that everything is known by the psychological action of the mind, so that philosophical speculation is the same as mental speculation if it is merely the random or haphazard activity of the brain to understand everything and making theories, “if’s” and “maybe’s.” But if philosophical speculation is directed by Sastra and Guru, and if the goal of such philosophical attempts is to achieve Visnu, then that philosophical speculation is not mental speculation. It is just like this: Krishna says in Bhagavad-gita that “I am the taste of water.” Philosophical speculation in the accepted sense then means to try to understand, under the direction of Sastra and Guru, just how Krishna is the taste of water. The points of Bhagavad-gita, though they are simple and complete, can be understood from unlimited angles of vision. So our philosophy is not dry, like mental speculation. The proper function of the brain or psychological activity is to understand everything through Krishna’s perspective or point-of-view, and so there is no limit to that understanding because Krishna is unlimited, and even though it can be said that the devotee who knows Krishna, he knows everything, still, the philosophical process never stops and the devotee continues to increase his knowledge even though he knows everything.
Ref >> Srila Prabhupada Vani.