Mysticism ('Irfan in Arabic), which etymologically means a kind of knowledge and perception, can be defined technically as a kind of immediate knowledge and understanding beyond the senses or reason. This is achieved through a process of overcoming obstacles and lifting the veils of the heart through spiritual wayfaring. In this way the mystic understands the inner truth, the hidden and the true Unity- that is hidden behind appearances and diversity. This process follows a spiritual journey and demands the perfection of the soul and is accomplished through self-knowledge and the enlightenment of the interior. Although from a historical point of view, what has firstly been called 'Irfan and expressed in terms such as "people of Sufism" can have five meanings. In the present work, the theoretical dimension of the specific meaning: "'Irfan as a set of instructions and teachings" have been primarily considered. It is known as the intellectual legacy of Islamic culture, and under the title "theoretical mysticism" it has been discussed from different perspectives and explained in prose, verse and in technical terms or symbolic words and concepts. This book is a textbook for Islamic Mysticism at postgraduate level; nevertheless, others who are interested in the subject may gain useful insights from the book.