Hetucakra or Wheel of Reasons is a Sanskrit text on logic written by Dignaga (c 480âÂÂ540 CE). It concerns the application of his 'three modesâ (trairà «pya), conditions or aspects of the middle term called hetu ("reason" for a conclusion) or linga ("mark", "sign" of a sound argument) in a valid inference within the Indian logico-epistemic tradition, sometimes referred to as Buddhist logic.
Anacker (2005: p. 34), in introducing his English rendering of the "Method for Argumentation (VÃÂda-vidhi)" of Vasubandhu (fl. 4th century)âÂÂa text composed in Sanskrit which is now only extant in a reconstructed composite extracted from Tibetan works, collated by Frauwallner (1957)âÂÂholds that: <blockquote> Vasubandhu's criteria for a valid inference-schema are concise and precise, and there is nothing essential omitted. DignÃÂga's 'wheel of justifications' (hetu-cakra), sometimes held to be the first complete Indian formulation of what constitutes the validity and invalidity of an argument, is in fact nothing of the kind: it is a pedagogic device mapping out in detail what Vasubandhu's criteria already presuppose. </blockquote>
Dignaga formulated the 'three modesâ (trairà «pya) which are three conditions required for a logical âÂÂsignâ or âÂÂmarkâ (linga), which to fulfill in order to establish the 'valid cognition' (pramana) of an 'inference' (anumana):
When a âÂÂsignâ or âÂÂmarkâ (linga) is identified, there are three possibilities: the sign may be present in all, some, or none of the sapaká¹£as. Similarly, the sign may be present in all, some or none of the vipaká¹£as. To identify a sign, by convention we accept the first condition as being satisfied.
Combining these, Dignaga constructed his âÂÂWheel of Reasonsâ (Hetucakra) with nine distinct possibilities, which may be tabulated as follows (adapted from Matilal, 1998: p. 9):
Of the nine possibilities within the cakra or 'wheel', Dignaga asserted that only two are illustrative of sound inference, that is they meet all three conditions, namely Numbers 2 and 8: either "+ sapaká¹£a & â vipaká¹£a" or "ñ sapaká¹£a & â vipaká¹£a" would fulfill the required conditions. Dignaga is insistent that at least one sapaksa must have the positive sign. Number 5 is not a case of sound inference as this is a pseudo-sign for although it satisfies the two conditions 1 and 3, it does not fulfill condition 2.
Dignaga required all three conditions to be met in order to establish valid cognition. The second row does not satisfy condition 2 and hence none of Numbers 4, 5, and 6 are logical signs; they are pseudo-signs. Numbers 4 and 6 are called âÂÂcontradictoryâ pseudo-signsâÂÂan improvement upon the old NyÃÂya Sà «tras definition of contradictory. The middle one, Number 5, is called âÂÂuniquely deviatingâ (asÃÂdhÃÂraá¹Âa), perhaps for the reason that this sign becomes a unique sign of the paksa itself, and is not found anywhere else. In Dignaga's system, this sign cannot be a sign for anything else, it can only point to itself reflexively or to its own locus. Numbers 1, 3, 7, and 9 are also pseudo-signs. They are called the âÂÂdeviatingâ signs, for in each case the sign occurs in some vipaksa or other, although each fulfills the second condition. This shows that, at least in Dignaga's own view, the second condition (when it is combined with the first) gives only a necessary condition for being an adequate sign, not a sufficient one. In other words, Dignaga intended all three conditions jointly to formulate a sufficient condition.