rivulets or in sheets, will enable us to infer that the flood was due to rains and not to the rise of water in the river.  Thus we see that Nyaya relied on empirical induction based on uniform and uninterrupted agreement in nature, whereas the Buddhists assumed a priori principles of causality or identity of essence.  It may not be out of place here to mention that in later Nyaya works great emphasis is laid on the necessity of getting ourselves assured that there was no such upadhi (condition) associated with the hetu on account of which the concomitance happened, but that the hetu was unconditionally associated with the sadhya in a relation of inseparable concomitance.  Thus all fire does not produce smoke; fire must be associated with green wood in order to produce smoke.  Green wood is thus the necessary condition (upadhi) without which, no smoke could be produced.  It is on account of this condition that fire is associated with smoke; and so we cannot say that there is smoke because there is fire.  But in the concomitance of smoke with fire there is no condition, and so in every case of smoke there is fire.  In order to be assured of the validity of vyapti, it is necessary that we must be assured that there should be nothing associated with the hetu which conditioned the concomitance, and this must be settled by wide experience (bhuyodars’ana).

Pras’astapada in defining inference as the “knowledge of that (e.g. fire) associated with the reason (e.g. smoke) by the sight of the reason” described a valid reason (li@nga) as that which is connected with the object of inference (anumeya) and which exists wherever the object of inference exists and is absent in all cases

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where it does not exist.  This is indeed the same as the Nyaya qualifications of pak@sasattva, sapak@sasattva and vipak@sasattva_ of a valid reason (hetu).  Pras’astapada further quotes a verse to say that this is the same as what Kas’yapa (believed to be the family name of Ka@nada) said.  Ka@nada says that we can infer a cause from the effect, the effect from the cause, or we can infer one thing by another when they are mutually connected, or in opposition or in a relation of inherence (IX. ii. 1 and III. i. 9).  We can infer by a reason because it is duly associated (prasiddhipurvakatva) with the object of inference.  What this association was according to Ka@nada can also be understood for he tells us (III. i. 15) that where there is no proper association, the reason (hetu) is either non-existent in the object to be inferred or it has no concomitance with it (aprasiddha) or it has a doubtful existence sandigdha).  Thus if I say this ass is a horse because it has horns it is fallacious, for neither the horse nor the ass has horns.  Again if I say it is a cow because it has horns, it is fallacious, for there is no concomitance between horns and a cow, and though a cow may have a horn, all that have horns are not cows.  The