Dear Prof. Olivelle,
Thanks for your email. You are absolutely right. When I was writing my thesis, I did think very carefully about whether I should use the terms 'left' and 'right' while talking about pūrva and para respectively.
One reason, of course, was, as you mention, that these terms only makes sense in the written context. The other reason for my apprehension about the accuracy / appropriateness of these terms was that, if Sanskrit were to be written, like Persian, from right to left, then para would have meant 'left'.
So, why did I eventually decide to use the term 'right'? The thing is, I had to make it absolutely clear that, according to me, paraṁ kāryam means 'the rule which is applicable to the part that is spoken / pronounced later in time'. For example, the suffix is uttered / pronounced later than the base. I realized I would find it difficult to use this long and somewhat awkward phrase ('the rule which is applicable to the part that is spoken / pronounced later in time') again and again in my thesis. And since I've introduced so many new ideas in my thesis, I wanted to present things diagrammatically wherever possible to help the reader understand what I am trying to say. For all these reasons, I decided to translate para as right hand side when translating not only (my interpretation of) 1.4.2 vipratiṣedhe paraṁ kāryam, but also in other rules such as 1.1.51 uraṇ raparaḥ and 1.1.47 mid acho'ntyāt paraṇ.
I have not provided the above clarification in my thesis. In hindsight, I think I definitely should have. In the future, when I write about my doctoral research, I will most certainly do so. Thank you very much for bringing this up.
In my recent interviews though, including the one with BBC radio yesterday, I did explain that paraṁ kāryam actually means 'that rule which is applicable to the part (to be more precise, operand) which is spoken / pronounced later', and that I use the term right hand side purely because I find it convenient to do so.
Best,
Rishi
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Dr. Rishi RajpopatMSt (Oxford), PhD (Cambridge),
Former Gonda Fellow (Leiden)