Understanding the Question: The question asks to identify the philosopher-grammarian who composed the Vākyapadīya, a seminal work that shifts the focus from mere grammatical rules to the deeper philosophy of language.
Key Concepts and Approach: The Indian grammatical tradition (Vyākaraṇa) evolved from the formal rules of Pāṇini to the philosophical inquiries of Bhartrhari. The Vākyapadīya is famous for treating the "sentence" (Vākya) and the "word" (Pada) as central units of meaning and metaphysical reality.
Detailed Solution:
Step 1: Analyzing the Authorship. While Pāṇini wrote the rules (Sūtras), and Patañjali wrote the great commentary (Mahābhāṣya), it was Bhartrhari (c. 5th century CE) who synthesized these into a linguistic philosophy in his work, the Vākyapadīya.
Step 2: Examining the Structure. The text is divided into three books (Kāṇḍas): the Brahmakāṇḍa (metaphysics of language), the Vākyakāṇḍa (theory of sentences), and the Prakīrṇakāṇḍa (miscellaneous linguistic topics).
Step 3: Identifying Core Theories. Bhartrhari is most renowned for the Sphoṭa theory, which posits that meaning is revealed through a unified linguistic burst rather than a sequence of individual sounds.
Conclusion: Therefore, the author of the Vākyapadīya is Bhartrhari.