In the comments, Franklin Mason questioned whether my criteria for meaning meant that meaning was infinitely complex:
The set of possible experiences that might serve as verification of some existential proposition is infinite. Thus if an existential proposition means that set of possible experiences, that proposition is infinitely complex. But if it is infinitely complex, it cannot be grasped by minds such as ours. We do however grasp many simple existential propositions. Thus . . .The problem with this critique is that an infinite enumeration of cases does not make a proposition infinitely complex.
If I say that X is greater than 3 and less than 4, there are an infinite number of points I could enumerate that would satisfy (or violate) my proposition, but the proposition is not infinitely complex. This is because it is simple to execute the test of the proposition's veracity. The proposition would only be infinitely complex if the execution of the verification test for the proposition were infinitely complex.
Franklin also echoed Bob's earlier comment on what constituted detectability:
When you say 'detectable', to whom do you mean? Only humans? Surely not. What organs of sense can be put to use? Only those we have? Again, surely not. I don't see clearly what a good account of 'detectable' would come to.As with Bob's criticism, the answer is that detectability does not have to be direct, but must at least be indirect.
I have never seen my friend's girlfriend, but she has been described to me. Thus, I have no direct experience of her, but I have indirect experiences such that propositions about her are verifiable, by other indirect or indirect experiences.
Finally, Franklin asks whether possible experiences are themselves undetectable:
You speak at times of possible experiences. What sense can an empiricist such as yourself make of that talk? Are there possible nonactual things? You seem to assume that there are. But surely such things are undetectable . . .This critique is more subtle, and it illustrates the way in which meaning can get lost when we try to generalize away from verifiability.
Suppose I say that "Roses are red." I making this claim, I am contemplating several possible experiences. These possible experiences are experienced by me now. Possibilities are themselves actualities in thought. I have created real, mental images of roses of different colors that I will compare with physical roses at a future time. These possible roses exist as models now, before I ever observe a rose in color. So it would be incorrect to claim that, for the purposes of making meaning for propositions, possible experiences are undetectable. Rather, they are the vital actualities upon which meaning is based.