Patanjali in his Yoga aphorisms says that it is the obserser-observed phenomenon that is the cause of man's suffering. Modern physics too is concerned with the perceiver's intrusion into every act of perception. And surely the perceiver is the mind. In its very functioning the problem of duality inheres. And where duality is, right perception cannot be.
Right perception demands non-duality.
The non-dual perception is the very core of mysticism. The entire upanishadic teachings are based on the non-dual experience. Evidently HPB gave her last gift to the world, The Voice of the Silence only to benefit the few individuals, the real mystics in the Theosophical Society. There are all sorts of notions about mysticism. Some identify it with devotion, others with vague emotionalism.