To follow up the post I made just a few minutes ago, about the difference between Jimmy punches and Jimmy punched Bobby and what θ-roles are involved, let me say one other thing about that.
We probably want to assume that the verb punch means something, and means something stable. And probably this basic meaning of punch is indeed transitive, with an Agent and a Theme.
It “feels” like the wrong way to go to suppose that English just has two different verbs that sound like punch, one of them being transitive and describing a particular type of contact between Agent and Theme, and another one that just describes the performance of a particular type of arm motion.
It is probably not an accident that the “intransitive” meaning of punch, describing a particular kind of arm-moving event, is related to the kind of arm-movement used in the “transitive” meaning of punch. Moreover, it’s possible to do this kind of “detransitivization” with a great many transitive verbs (e.g., Jimmy eats, Jimmy paints), and it can’t be an accident that it always seems to retain a basic aspect of the meaning but lose its connection to any Theme.
What this leads us to is an analysis under which there is some way to derive the intransitive form from the transitive form. That is, punch in Jimmy punches is the transitive form punch plus some systematic (grammatical) modification that severs the Theme.
This is a little bit like what the passive does, and we’ll talk about the passive after the midterm. But briefly, if you have the active sentence Jimmy punched Bobby, the passive version would be Bobby was punched. What the passive does is remove the Agent from a transitive verb, and make it intransitive. With the passive, there is some visible morphology that goes along with it: the verb shows up in a participle form, and the verb be is used.
To account for Jimmy punches, we might want to say something similar, although this isn’t really going to be something we spend any time analyzing specifically in this class. The transformation of a transitive verb into an intransitive verb that is accomplished by removing the Theme is sometimes called an “antipassive,” where the other difference between passives and antipassives is that there is no audible indication (in English) that it has happened.
English does allow this sort of “inaudible transformation” in other cases. Probably the easiest one to see is in the use of nouns as verbs. In some cases, to turn a noun into a verb requires adding a suffix: glory → glorify. But it’s often comprehensible (and in many, many cases has become a regular part of the language) to simply verb a noun, as in: I sent Mary an email → I emailed Mary. Often we assume, parallel to glorify, that there’s a silent “verbing suffix” that converts email (N) to email (V). Given that, it might not be so crazy to think that there could also be a silent “antipassivization” suffix that converts punch (transitive) to punch (intransitive).