…because it is so difficult to determine whether they have a truth-value. This point is acknowledged in the contemporary debate on deontic logic:

A fundamental issue of deontic logic is Jorgensen’s dilemma, as noted by Jorgensen. On the one hand, there are inferences involving norm sentences such as ‘you should stay‘ or ‘you may leave‘ in our lives; therefore there should be a logic dealing with them. On the other hand, these sentences express orders or permissions and do not have tuth values: therefore, there cannot be such a logic. A dilemma arises. (Ju and Liang 2015, section 1)

Out of probably similar reasons, also within Indian philosophy almost no school focused on the logic of prescriptions. Even within the only one which did, Pūrva Mīmāṃsā, some authors then moved back towards the safer ground of understanding prescriptions as descriptions. Again, in the words of Ju and Liang:

To solve this dilemma, many philosophers have proposed a distinction between two different uses of norm sentences: descriptive and prescriptive uses. In the descriptive way, norm sentences are used to state what agents ought to do; they can be true or false. […] Deontic logic is ‘legalized’ in this way. (Ibid.)

In this sense, trying to “legalize” deontic logic is a way to deal with it and to attribute truth values to it. Kumārila went a little bit in this direction when he stated that prescriptions refer to the future (which is still beyond the precinct of application of truth values, but not as much as the deontic domain, which will never be). Maņḍana went much further and claimed that, e.g.,

O x / you desire y (“You ought to do x if you desire y”)

is tantamount to:

x is a means to realise y

Why so? Because of the dilemma mentioned above, but probably also because Maṇḍana was in part closer to Vedānta than to Pūrva Mīmāṃsā and was in this sense keen to avoid the commitment to sādhyavākyārthavāda, i.e., to the theory according to which all sentences can only convey a prescriptive meaning.

I am grateful to Bama Srinivasan, who sent me a copy of Ju and Liang’s article.