Kevin R. Gregg is Professor Emeritus at Momoyama Gakuin (St. Andrew's) University in Japan, and the most consistent philosophical voice inside SLA for treating theory construction as a serious obligation rather than a rhetorical choice. He trained as a linguist, read widely in philosophy of science, and spent the decades after the 1984 "Krashen's Monitor and Occam's Razor" paper arguing that if you want to claim to have explained anything about language acquisition you need a property theory of what language is, and you need to mean it.
The arguments are sharp by design. Gregg defends Chomskyan UG as the best property theory the field has, and he has been the most visible critic of emergentist and strongly social-turn accounts on the grounds that they keep trying to explain language learning without first saying what is being learned. His 1996 "Logical and Developmental Problems of Second Language Acquisition" is the standard citation for the position, and his 2003 "The state of emergentism in SLA" is the most-cited attack on usage-based rivals.