“It is impossible that evolutes do not arise from one single cause, because [if this were not the case] it would follow that they would differ [from each other completely]” (see Watanabe’s reconstruction of the argument at p. 1230: na vyaktasyaikapūrvakatvābhāvaḥ, bhedaprasaṅgāt).

It is distinguished from the vīta ‘direct’ argument, e.g.:

“A primordial matter exists, because one sees homology* among the various [evolutes]” (asti pradhānam, bhedānām anvayadarśanāt, see p. 1230).

Watanabe shows how Diṅnāga criticises the difference between the vīta and the āvīta syllogism, claiming that in fact there is no difference between the two reasons (i.e., anvaya and bhedaprasaṅga (ko hy atra viśeṣo bhedaprasaṅgānvayayoḥ, PSV 3.16). Watanabe shows in logical notation that the point is that for Sāṅkhya ~anvaya is tantamount to bheda: thus the two arguments become identical.
However, Dignāga does not just criticise the Sāṅkhya argument, he re-interprets it by means of embedding the prasaṅga way of reasoning (what we would call reductio ad absurdum) into his trairūpya analysis of the syllogism. In this way, the reductio ad absurdum, that was probably until that point a dialectical device used against one’s adversaries in debates, became part of the logical structures of legitimate syllogisms.

Accordingly, Watanabe can aptly show (p. 1232) that the structures of Dharmakīrti’s prasaṅga and prasaṅgaviparyaya arguments (two types of reductio ad absurdum) correspond to Dignāga’s analysis of the vīta and āvīta arguments.

In other words, the substitution of prasaṅga instead of āvīta is one of the instances in which one can observe how the previously dominant epistemological school, the Sāṅkhya, has been replaced by the Buddhist epistemological one.

(cross posted also on The Indian Philosophy blog.)

*I am indebted to Toshikazu Watanabe for a valuable comment on my translation of anvaya. I had originally translated it as “continuity” for etymological reason (anu-i– `to recur’) and because of the meaning it assumes in the later context of the syllogism. Watanabe suggests that what is meant with anvaya is, instead, better rendered with “homology”, which “ means, like in the context of biology, the similarity or correspondence of nature between things that have a common evolutionary origin”.