The Ocean of Inquiry is a vernacular compendium of Advaita Vedānta, one of the most influential traditions of South Asian religion and philosophy, especially in modern times. Its author, Niścaldās (ca. 1791 – 1863), was a classically trained pandit and a sādhu of the Dādū Panth. His work was widely read in the nineteenth and early twentieth centuries, both in its Hindi original and in regional translations: Vivekananda once referred to it as the most influential book in India. Surprisingly, however, The Ocean of Inquiry remains virtually unknown to Western scholars; even specialists in Hinduism have rarely heard of it. This dissertation aims to draw attention both to Niścaldās’s work and to the broader genre of vernacular Vedānta; it also calls into question the notion that late Advaita Vedānta represents a period of intellectual decline. Part I provides a historical and textual overview of The Ocean of Inquiry, arguing that Niścaldās’s work should be situated within what might be termed "Greater Advaita Vedānta," or Advaita Vedānta as it was disseminated outside the received canon of Sanskrit philosophical works. This part of the dissertation also offers the first comprehensive biography of Niścaldās in English, and it analyzes the significance of his choice to write in the vernacular. Part II investigates the relationship of philosophy and religious practice in Niścaldās’s work. Taking as its starting point the question "What does it mean for knowledge to liberate?" this part of the dissertation argues that for Niścaldās, the key distinction is not between theoretical knowledge and liberating knowledge but between doubtful and doubt-free awareness. For those who are properly qualified, the central practice on the path to liberation is the practice of inquiry (vicāra), interpreted as a dialectical process of raising and removing doubts. This interpretation is supported with three "case studies" of characters in The Ocean of Inquiry who reach liberation. The conclusion is that for Niścaldās, philosophical inquiry is not a purely theoretical undertaking; under the right conditions, it can become a concrete religious practice.