The different avenues that lead to spiritual understanding can be understood best through an initial distinction between spirit and matter. In order to understand matter we have material means, and in order to understand the spirit we have spiritual means. Matter is understood through the mind or the intellect working upon data given through the different senses, but spirit can be understood only through the spirit itself. This highest form of understanding, in which the spirit enjoys Self-knowledge without using any instrument or medium, is very rare and most difficult to attain. The best approach for the understanding of the spirit is through the heart and not through the mind.
The mind is accustomed to work upon material things, and its driving power for this intellectual understanding of material objects is derived from lusts and cravings. When mind is turned toward spiritual problems, it tackles them along lines it is accustomed to; and in so doing, mind uses concepts it has invented for the intellectual understanding of material things. However, this approach to understanding spiritual problems is doomed to fail, because all concepts that the intellect evolves for knowledge of material things are inadequate for understanding the spirit. It is like trying to see through the ears or hear with the eyes. If the mind tries to understand the spirit independently  of the heart, it is bound to use analogues from the material world; and this inevitably leads to the spirit being looked upon as an object of the mind, which it is not.
– Discourses 7th Ed. p94