Udayana, (Devanagari: उदयन) also known as Udayanācārya (Udyanacharya, or Master Udayana), (circa 975 - 1050 CE[2]) was an Indian philosopher and logician of the tenth century of the Nyaya school who attempted to devise a rational theology to prove the existence of God using logic and counter the attack on the existence of God at the hands of Buddhist philosophers such as Dharmakīrti, Jñānaśrī and against the Indian school of materialism (Chārvaka).

For example, Gaṇgeśa Upādhyāya, the 14th century Indian philosopher and mathematician who established the Navya-Nyāya school refers to Udayana as "Acāryaḥ" (lit.

[9] The controversy about Udayana's lifetime seemed to have been settled by the discovery of the Lakṣaṇāvalī, the concluding verse of which states that it was written in Saka era 906 (984—985 CE).

[10] Udayana's work represents a synthesis of ontological and epistemological theories, combining the Vaisheshika system of categories (padarthas) with the Nyaya doctrine of pramanas (means of knowledge).

He aimed to establish clear boundaries for each concept, which he believed was essential for maintaining logical consistency and clarity in philosophical discourse.

[11] Udayana distinguished philosophy from spiritual or religious practices by emphasizing its foundation in logical reasoning and epistemology.

This approach contrasts with other Indian philosophical traditions that often intertwine metaphysical inquiries with spiritual goals like moksha (liberation).

His work is characterized by a rigorous analytical approach, emphasizing the importance of clear conceptual distinctions and logical analysis in the pursuit of truth.