The mind (meaning the intellectual mind) is—in and of itself and on its own level—not capable of apprehending the real, where the “real” here refers to spiritual reality. Spiritual reality is above or transcendent to the mind and the mind can not, by analysis, memory, etc, gain access to the truth in the spiritual sense.
Consequently when one is focused only at the mental or intellectual level, that activity can block, screen out, discount—in effect “slay” the real. It does not, of course, literally “slay” anything, rather it means that mind can act as a barrier to perception of the deeper truths of life.
This does not mean that the mind should not be fully developed, educated, and sharp. Blavatsky was herself a great intellect. It’s just that there is far more to man than intellect. The intellect is a subsidiary aspect of the higher consciousness, or put another way, it is only one aspect of conscious a partial expression or extension of the soul, and not the whole of what man is.
True knowledge, in the spiritual sense, resides in the soul, the spirit, the higher self. The intellect deals well with the physical world, the facts given by the external senses, and related matters. But, by evolution, the eventual function of the mind also includes the ability to mirror spiritual reality and transmit and adapt it at the intellectual level.
In the related writings of Alice Bailey there is a version of the same concept in these words: “When the fire of the mind burns too brightly, it shuts out the light of the soul.” But the key thought here is not just “brightness” but rather that psychological condition in which the intellect becomes the exclusive ruler of the life. This blocks realization of the heart or soul aspect, of spiritual love and higher consciousness. When the intellect usurps all the power it also blocks spiritual intuition that reveals the meaning of life, and much else besides.
A good example of this type of intellectual “slaying the real” is found in “scientism” as distinct from science itself, for which see The Folly of Scientism