The meaning of causative 'have'

I’m a bit loath to offer an answer when there are so many erudite comments, but I’m interested in the topic and can’t comment yet, so here goes.

I think the explanation is semantic - besides the non-causative sense illustrated by Bill had his tires slashed last night, there seem to be two causative senses of had, roughly (1) to exercise authority over someone such that they use their agency in a particular way (2) to bring about some situation through your own agency. I think Post’s first example invites sense (1), while his second invites sense (2), as do both of Huddleston & Palmer’s – so naturally Post judges his sentences to be different, while Huddleston & Palmer judge theirs to be the same.

The scenario Post’s first sentence conjures up for me is that Tom is some sort of lead investigator and has authority over Melvin, which he exercises by getting Melvin – as opposed to someone else on the team – to conduct the interrogation. That’s sense (1).

Huddleston & Palmer’s first sentence is different in that we have no reason to suppose the father has any authority over the specialist. For one thing, that’s not how the world works – while investigators generally have superiors who get to tell them what to do, there isn’t usually a comparable relationship between specialists and the fathers of their patients. For another thing, we are invited to relate the father to the son rather than the specialist, both for the obvious reason and because the reference is to a specialist and not a specific person. All of that points to sense (2).

I think Post’s second sentence is also an example of sense (2). An interrogation is not normally something we undergo voluntarily, so if Tom has power to force the witness into it, it’s a coercive power. At the point where you are coercing someone into something, their agency is no longer in play, which means it can only be sense (2). In any case, I read H&P as saying that had never has sense (1) in the passive construction, and as I say below I think they're right.

On that basis, Post’s two examples are using had in two different senses (of course, all that is necessary to explain his statement is that they can plausibly be read as using it in two different senses).

As for Huddleston & Palmer’s second example, it’s quite plausible that the father would have authority over the son, so either sense could be in play, and maybe it comes down to whether you think that the passive construction is compatible with sense (1) – or maybe it’s enough to point out that H&P don’t. At least, that’s how I read p. 1236, where they say he got his son to be examined by a specialist is not an alternant of he got his son examined by a specialist, and has no equivalent using have. As far as I can see, the first of those sentences expresses the same thing as my sense (1), and what H&P are saying is that there is no passive sentence with have that does the same job. If that’s how they see it, they’re bound to read both of their sentences in sense (2), and judge them to be the same.

For what it’s worth, I think H&P are right – you can come up with active sentences with have and got that are equivalent and express sense (1) – e.g. he had his son tail the specialist / he got his son to tail the specialist - but I’ve drawn a blank as far as passive sentences are concerned. Is it possible that the raising construction requires more arguments than are licensed by this use of have?