प्रधानप्रत्ययार्थवचनमर्थस्यान्यप्रमाणत्वात्

(Nor need be taught) the rule relating to the dependance of the meaning of a word on the principal (प्रधान in a compound) or on the affix (प्रत्यय) because the authority of the meaning (of a word, compound or derivative) consists in something else.
This sutra may be divided into two parts. The first part stating the proposition and ending with the word vachanam. The second part giving the reason. The first part consists of four words; Pradhana: 'Principal in a compound.' As in the compound राजपुरुषः 'king's man,' the word raja is Upasarjana; and पुरुषः is pradhana. 2. Pratyaya: 'affix.' 3. 'meaning.' 4. वचनम् 'word or sentence.'

In the time of Panini, there were some grammarians who held the opinion, that the meaning of a word is determined, if a compound word, then by its principal word; and if a derivative word, then by the suffix. Panini controverts this opinion by declaring that in as much as the meaning of a word is fixed by usage and idiom, therefore the proofs for the meaning of a word are not to be searched either in the affixes which go to form it; or by its principal word, if it is a compound. Because, says Panini, the proof of the meaning of a word is in something else, that is to say in the usage of a people and not in the suffixes, &c. A person who has never heard the name of grammar, understands full well when told to bring a Raja-purusha. He brings an official and not a king, nor even any person in general. Similarly when told to bring an औपगव he brings a cowherd's child, not a cowherd, nor a child in general, nor both. When, therefore, the sense of a word is determined by usage, there is no use to strive to find out the sense by grammatical rules.