B.S.1.1.4.S.B.--- nanu jnaanam naama maanasii kriyaa. na, vailakshaNyaat-------- veditavyam. jnaanam (knowledge) is not a mental act, because there is a difference (between knowledge and meditation). A mental act is seen to exist where there is an injunction about it, which is independent of the nature of the thing concerned. dhyaanam (meditation), is a mental act, because it depends on the will of the person performing it.
For example, to think of a man or woman as fire, as enjoined in " O Gautama, man is surely fire" (Ch.up.5.7.1) , or in "O Gautama, woman is surely fire" (Ch.up.5.8.1) is certainly a mental act, since it arises from an injunction alone. But the idea of fire with regard to the well-known fire is not dependent on any injunction or on the will of any
man. (In other words, thinking of one thing as another, like a linga as Lord S'iva and worshipping it as such, is meditation and it is a mental act, because it depends on the
will of the worshipper. But looking at an ordinary stone and seeing it as a stone is knowledge and is not a mental act, because it does not depend on the will of the
person). While meditation depends on the will of a person, knowledge depends only
on the object concerned and on valid means of knowledge, such as perception.
Meditation is therefore described as purusha-tantra (dependent on the person),
while knowledge is called vastu-tantra (dependent on the object to be known).