 Vyāpti or pervasion, is that relation between the inferential sign (hetu) and the inferred property (sādhya), which legitimises the inference. It would typically be expressed by a sentence such as “wherever there is smoke there is fire”, or “whatever exists is transitory”. Knowledge of this relation, according to the Nyāya, is the instrumental cause in the inferential process—it is that relation knowledge of which, when combined with observation of the inferential sign, will permit us to make a sound or knowledge-yielding inference. Gaṅgeśa therefore attaches great importance to the precise definition of this relation. He notes as many as twenty-one definitions all of which he rejects for some reason or other, and then he goes on to give seven further formulations, each of which he considers acceptable. Of the definitions he rejects, the first five came to be known as the ‘vyāpti-pañcaka’, and inspired a huge literature both among the Sanskrit commentators and their modern interpreters. These definitions are traceable to the earlier Buddhist and Nyāya literature. Two more rejected definitions, known as the ‘Lion and Tiger’ definitions, are apparently due to Gaṅgeśa’s Navya-Nyāya predecessors (Wada 2007, ch. 5). The definition Gaṅgeśa finally accepts is called his ‘siddhānta-lakṣaṇa’.