Did Buddha acknowledge "reductio ad absurdum" argumentation & other apriori forms of reasoning in Buddhist logico-episteomology (looking for sources)?

Question

It is well known that many Buddhist philosophers and scholars of various traditions, such as Nagarjuna and Chandrakirti rely on and acknowledge reductio ad absurdum as a method of positing arguments and discussing different forms of knowledge.

On the Wikipedia page regarding Buddhist logico-epistemology, it is stated:

K. N. Jayatilleke sees Buddha's epistemological view as a kind of empiricism which also includes a particular view of causation (dependent origination): "inductive inferences in Buddhism are based on a theory of causation. These inferences are made on the data of perception. What is considered to constitute knowledge are direct inferences made on the basis of such perceptions."[13]: 457  Jayatilleke argues the Buddhas statements in the Nikayas tacitly imply an adherence to some form of correspondence theory, this is most explicit in the Apannaka Sutta (MN 60).*He also notes that Coherentism is also taken as a criterion for truth in the Nikayas, which contains many instances of the Buddha debating opponents by showing how they have contradicted themselves.[13]: 352–353.  He also notes that the Buddha seems to have held that utility and truth go hand in hand, and therefore something which is true is also useful (and vice versa, something false is not useful for ending suffering).[13]: 359 

However, on that same page, it is stated:

According to Jayatilleke, 'pure reasoning' or 'a priori' reasoning is rejected by the Buddha as a source of knowledge.[13]: 273  While reason could be useful in deliberation, it could not establish truth on its own.

This is slightly confusing to me because if you are subscribing to some of the views mentioned in the first paragraph (and some other Buddhist concepts more generally), there is no way to defend them in a purely posteriori way. Thus, at least some apriori reasoning is necessary in order to even begin establishing other principles used throughout Buddhist philosophy and worldview which are considered truthful. Reductio ad absurdum is one such way of establishing truth in an apriori way.

As such, does Buddha see reductio ad absurdum as valid forms of argumentation and deriving truth, and if not, how does he argue for different concepts without relying on apriori reasoning?

I just want to clarify that in this case, I am saying "reductio ad absurdum" to mean the following: reductio ad absurdum argument is showing that non-acceptance of some X would result into a complete incoherence and any non-acceptance would simply not be sound and coherent, and hence, it necessarily must be accepted for anything other than its affirmation is not possible. In other words, non-acceptance of some X concept or idea would result in a self-refuting idea logical fallacy. And as noted above, it seems that Buddha subscribed to the notion that contradiction is not acceptable. So I think it would only make sense if Buddha affirmed reductio ad absurdum for there can be situations where anything other than embracing and accepting some idea X would result in complete incoherence, and this is technically done in an apriori and deductive manner but according to Jayatilleke, Buddha rejected apriori argumentation so it doesn't make much sense.

I hope that what I wrote is clear. I'm a beginner and interested in learning Buddhist concepts. If something I wrote is incorrect, please let me know and I will change it.

Answer 1

All concepts which Buddha discusses are based on experience. There is no a priori reasoning involved. Buddha says come and experience yourself. All reasonings are rooted in experience. However there is “reductio ad absurdum” or proof by contradiction.

For example - Theory of Anatta or not-Self.

You do not begin by assuming Self. But you begin by asking , what is that which you call self ? Is it forms ? Or is it feelings ? Or is it perceptions? Or is it choices ? Or is it consciousness?

Then the question arises , Is form permanent or impermanent? Are feelings permanent or impermanent? Are perceptions permanent or impermanent? Are choices permanent or impermanent? Is consciousness permanent or impermanent?

The answer is ,impermanent ,for all the above because all conditioned phenomena are impermanent. It is an observed fact of existence which can be experienced by anyone.

Then we argue that , if form is impermanent then is it suffering or not ? If feelings are impermanent then is it suffering or not ? If perceptions are impermanent then is it suffering or not ? If choices are impermanent then is it suffering or not ? If consciousness is impermanent then is it suffering or not?

The answer is yes, it is suffering. If the answer is yes then is form , feelings , perceptions, choices and consciousness are worth calling Self?

The answer is no. Therefore all conditioned (and unconditioned) phenomena are not-self.

What we assumed as Self turns out to be not-Self.

Answer 2

I guess an example of "proof by contradiction" is in the 2nd or 3rd sutta, i.e. Anattalakkhaṇasutta (SN 22.59)

The Buddha said this:

“Mendicants, form is not-self. For if form were self, it wouldn’t lead to affliction. And you could compel form: ‘May my form be like this! May it not be like that!’ But because form is not-self, it leads to affliction. And you can’t compel form: ‘May my form be like this! May it not be like that!’