Introspection - What is it?

The self-existent Soul created the mind and senses with outgoing tendencies. The mind has a pernicious habit of externalization from time immemorial. So you behold the external universe and not the internal Self. It is the Maya (Delusion) that draws you out. From your childhood, you are taught to look to the external world and not to the internal, psychic world. You have entirely lost the faculty of introspection. To have a comprehensive understanding of what is going on in the inner 'mental factory', a Pure reasoning and subtle intellect with power of introspection is needed. You will have to turn the mind inside, then concentrate all its powers and throw them upon the mind itself, in order that it may know its own nature and analyse itself. This is Raja Yoga (royal path of yoga).

Make a vigorous and earnest search within. Do not trust the mind and the senses. They are your enemies. Attachment to material world and people are your bitter foes. In introspection, the mind itself is the subject of study. A portion of the mind studies the remaining portion of the mind. The higher mind studies the lower mind. Introspection is apperception. Just as you watch the work done by a labor, a portion of the mind watches the movements of the rest of the mind. If you are one with the mind, if you identify yourself with the mind, you cannot know your defects. If you are a silent witness of the mind and if you practise introspection, you can know your various defects.

By a careful watch, many defects are detected and removed by suitable practice. Enter a quiet room. Enter into silence daily for about fifteen minutes, morning and evening. Introspect. Watch the mind carefully. The mind will be doing either thinking, planning, feeling, knowing or willing. You will have to find out through subjective introspection what the mind is exactly doing at a particular time. To go through this practice, you must have a subjective mind and a subtle Intellect. Intellect can be rendered subtle by study of philosophical books, control of senses and Sattvic food. Constant Mediation, Mantra chanting, Prayer purifies the mind-stuff and helps make the mind introspective.

Reference

Mind – Its Mysteries and Control by Sri Swami Sivananda