Hindu Mysticism is a collection of six lectures presented by S. N. Dasgupta at Northwestern University in 1926. The lectures cover six types of mysticism that grew up in India during the course of its history. Dasgupta defines "mysticism" as "the belief that the highest reality or the ultimate realisation and fulfilment...cannot be attained by reason alone, but...[by] the firm and steady control of will, the development of right emotions, or both combined, or by them both along with the highest functioning of reason....It is directed to the liberation of the spirit and the attainment of the highest bliss." This is distinct from what Dasgupta calls the popular notion that mysticism is "some kind of superstitious awe or reverence." Mysticism "is fundamentally an active, formative, creative, elevating and ennobling principle of life." It is "a spiritual grasp of the aims and problems of life in a much more real and ultimate manner than is possible to mere reason."
With this definition, most everything that can be understood as religion in India would be mystical and indeed that pretty closely describes the scope of Dasgupta's book. Its six chapters cover sacrificial mysticism, the mysticism of the Upanishads, Yoga mysticism, Buddhistic mysticism, classical devotional mysticism, and popular devotional mysticism.
Sacrificial mysticism is expressed in the ancient Vedas which describe how rituals can be performed that will prompt the gods to bestow benefits on those conducting the ritual. It is critical that the ritual be performed in exactly the right way or it will not be effective. Furthermore, if it is performed effectively, the gods have no choice but to bestow the benefit; that is, in response to a properly conducted sacrifice, the god's actions are not free. The Vedas are not seen to be true or effective because of a sagacious author or even as revelations from a god, but are "eternal truths, beginningless and immortal" and importantly, they cannot be challenged or justified by reason.
Upanishadic mysticism seems a bit more consistent with a European notion of mysticism, namely, a recognition of a monistic reality that lies behind the pluralistic appearances of the phenomenal world, which can be understood neither rationally nor empirically. Yogic mysticism, while it may in some practices deny the monism of the Upanishads, is a practical development of the Upanishadic mysticism. It clearly falls within Dasgupta's definition in that it is a practice that involves controlling one's will and actions, and ultimately one's mind to achieve an understanding of the ultimate reality that is not accessible through experience or reason alone.
Of all of the forms of mysticism described in Hindu Mysticism, Buddhistic mysticism might least fit with Dasgupta's definition. While it is true that achieving the highest understanding involves a practice rather like Yoga, Buddhism is a highly rational and practical system of thought. The fundamental principles of Buddhism should be apparent to anyone who follows the reasoning of the Four Noble Truths. The practice that is involved in following the Eightfold Path merely confirms those truths in one's experience. Nonetheless, it does not seem inappropriate to describe Buddhism as a mystical philosophy in that by following the Eightfold Path, one arrives at a state of consciousness that transcends normal experience.
The final two forms of mysticism, classical and popular devotional mysticism are closely related. Both embrace bhakti or devotion to God and in both cases one is not expected to denounce one's desires. Instead, one embraces the euphoria that comes in one's surrender to God. In the classical version, God is conceived of abstractly. The ecstasy that comes to the worshiper is similar to that experienced by Moslems, particularly, Sufis. In the popular version, one conceives of God in the form of an individual, e.g., Krishna, and the ecstasy is similar to that experienced in a human love relationship. As Dasgupta describes it, it seems similar to the mysticism of medieval Carmelites in their relationship to Jesus.
If there is a shortcoming to Hindu Mysticism it is its title. While there is nothing wrong with establishing definitions for the purpose of describing a conceptual realm, Dasgupta's definition appears contrived to capture every religious movement that ever came out of India and may capture most every religion there ever was. Had Dasgupta placed greater stress on the concept of gaining knowledge or understanding apart from experience or reason, the title would have been better justified. As it is Hindu Mysticism is simply a serviceable introduction to six religious traditions in India.