The affix is optionally attached, in the sense of 'wishing' after a root expressing the object wished and having the same agent of the action as the wisher thereof.,
As, कृ 'to do'; चिकीर्षति = कर्तुमिच्छति 'he wishes to do'. This is the Desiderative proper. It is optional to employ the desiderative affix, the same sense may be expressed by a phrase. The root must also be simple and not compounded with any upasarga, when the affix सन् is to be added. Thus प्रकर्तुमैच्छत् = प्राचिकीर्षत्.
Why do we say कर्मणः 'after (a root expressing) an object'? The affix will not come after an Instrumental case. Thus गमनेनैच्छति; here there is no affix. But गन्तुमिच्छति will be जिगमिषति.
Why do we say समानकर्तृकात् 'when the subject of the verb to wish, is also the subject of the verb denoted by the object wished' ? Observe देवदत्तस्य भोजनमिच्छति यज्ञदत्तः. Here Yajnadatta is agent of the verb इच्छति; but he is not agent of the action भोजनं; hence there is no affixing.
Why do we say इच्छायाम् 'when the sense is that of wishing'? Observe कर्तुं जानाति 'he knows how to make'; here the sense being that of knowing, there is no affix.
The word वा 'optionally' shows that a sentence may also be employed to express the same idea. Thus कर्तुमिच्छति or चिकीर्षति.
The word धातोः being especially used in this sutra, shows that this सन् is an ardhadhatuka affix, while the सन् taught in the two preceding sutras, will not get this designation.
Vart: The affix सन् is added, when the sense is that of 'in imminent danger'; as शङ्के पतिष्यति कूलम् = पिपतिषति कूलम् 'the bank is in imminent danger of falling down'. So also श्वा मुमूर्षति.
Vart: There is no affixing of सन् after a verb which has alrcady taken सन् in the sense of wishing. As चिकीर्षितुमिच्छति. But the सन् of (III. I. 5, 6) not denoting wishing, we have जुगुप्सिषते, मिमांसिषते.,