The notion of an informative rigid designator does a lot of work in Swinburne's book. He says that properties are individuated by the informative designators that pick them out. As a consequence of this, it is a purely apriori matter whether one property is the same as another. (As you might expect, the fact that there are no true apriori claims about the relationship between mental and physical properties is taken to be a reason to reject type identity claims. (And since events are defined in terms of properties and their instantiations, we get a quick argument against certain familiar forms of token identity theories. Editorial note: the quickness of that argument is a reason to worry about his approach to these issues, not a reason to worry about the status of token physicalism. The fact that you can't take seriously the idea that token physicalism might be true even if type physicalism is false just means that you aren't trying very hard to get the notion of an event right.))
So, what are these bloody informative rigid designators? I fear that Swinburne's guidance isn't quite as helpful as it should be. Here are two ways of thinking about IRDs that won't suit Swinburne's purposes but does seem to satisfy his initial gloss on the notion:
IRD1: If I know the meaning of 'this' when I say 'This is F', I know that for any x, x = the semantic value of 'this' iff 'this is x' is true. Since I'd know a condition that's necessary and sufficient for determining whether any x meets this (trivially because I know that they'd have to be identical to the semantic value of 'this'), all RDs are IRDs.
IRD2: If I introduce a descriptive name such as 'Julius' by means of a description, such as 'The person who actually invented the zip', I'd know a condition that's necessary and sufficient for any x to be Julius. For any x to be Julius, x has to be identical to the actual inventor of the zip. This allows us to maintain some distinction between RDs and IRDs if we say that some such descriptive knowledge is necessary.
It's obvious that the first proposal isn't at all in the spirit of Swinburne's suggestion and pretty clear that neither way of introducing IRDs would suit his purposes. That's because it's supposed to be apriori whether two IRDs pick out the same thing and the case of descriptive names shows that it's quite possible to introduce a kind of RD where it's true that one thing is picked out in two ways without it being apriori that this is so.
It seems what Swinburne needs, then, is something stronger:
IRD3: If I know the meaning of an IRD, I know _all_ the conditions necessary and sufficient for its proper application, not just a condition that's necessary and a condition that's sufficient.
He says, "Since the informative designators of any physical properties are not logically equivalent to those of any mental properties (since there are different criteria for applying the designators), no mental property is identical to a physical property" (69).
He says, 'The criteria for being in pain are how the subject feels, and the criteria for brain and behavioral events are what anyone could perceive. And the same applies to any other pure or impure mental property' (70).
Notice that his argument that's supposed to establish that mental properties are distinct from physical properties because the IRDs for mental and physical properties (and, by extension, events) is that the criteria for _applying_ IRDs for the physical properties make reference to things that are publicly observable and the IRDs for the mental properties make reference to how things feel. Now, if the subject who knew the meanings for the relevant IRDs knew all the necessary and sufficient conditions for their applicability, it seems that they'd know _more_ than just the criteria of application. That just requires knowing a sufficient condition for the IRD's application. What the argument needs to establish non-identity, however, is both knowledge that, say, pain requires just that something feels a certain way AND ALSO that there's no physical property that's necessarily instantiated by virtue of things feeling a certain way.
So, there seems to be a slip. It seems that Swinburne's argument against physicalism starts with the plausible idea that we know the meaning of the IRDs we use to talk about the mental where the plausibility of that idea turns on thinking of IRDs on the IRD2 model. That won't support the argument he's offering, however, because that requires thinking of IRDs on the IRD3 model. If we're clear that that's the relevant notion we have in mind, then there's just no reason at all to think that ordinary speakers know the meanings of the IRDs they use to refer to the properties that they do.
Maybe the quick way of putting the point is this. It's plausible that we know conditions _sufficient_ for the application of 'pain' or 'C-fiber firing', but only because this knowledge doesn't require knowledge of the total set of necessary and sufficient conditions that have to obtain for the term to be properly applied. What we need to establish non-identity claims is knowledge that meeting the condition for successfully attributing 'pain' does not involve, inter alia, meeting the condition for successful of application of, say, 'C-fiber firing'. Swinburne's argument goes nowhere precisely because it's supposed to provide us with this knowledge. Instead, it subtly assumes that we have this knowledge in an attempt to establish a non-identity claim. Being slippery in how we talk about IRDs just obscures this fact.