Philippi's law refers to a sound rule in Biblical Hebrew first identified by F.W.M. Philippi in 1878, but has since been refined by Thomas O. Lambdin.[1][2]
Essentially, in Biblical Hebrew, sometimes the sound /i/ shifted to /a/, but the reason for this development was unclear or debated.[3] It is "universally supposed to be operative", according to linguists in the field, but criticized as "Philippi's law falls woefully short of what one would expect of a 'law' in historical phonology...."[4]
Some critics suggested that it might not even be a rule in Hebrew, but rather a sound rule in Aramaic.[5] Even Philippi, who mentions it in an article about the numeral '2' in Semitic, proposed that "the rule was Proto-Semitic" in origin.[6][7] Philippi's law is also used to explain the vowel shift of Proto-Semitic bint for daughter to the Hebrew word bat (בת) and many other words.[8]