From PhilPapers forum Philosophy of Mind:

2010-07-02
Dretske on seeing
Ah, that's clearer now -- thanks Matthew.  Alston says that one person can go validly while another goes invalidly from the same grounds to the same conclusion. 

I am still having some trouble, though, in figuring out how this point would transfer to perception.  You quote Alston as saying:
... no propositional or factive ground to serve as a ground rather than the experience itself. One who does take experiences to be essentially propositional attitudes will find the same problem with doxastic grounds. 
I half-agree with the first part of this: when I conclude on the basis of my visual experience of the chocolate that it is brown, I may have only my experience of the chocolate to go on.  Now suppose (as I think) that the experience is indeed a propositional attitude: the experience that leads me to the belief that the chocolate is brown has content "That is brown".  

This is where I get stuck.  Are there supposed to be multiple routes from the perceptual experience to the conclusion?  (Perhaps there are, but there is only one instinctive route, I think.)  You say that this undermines the connection with the phenomenology.  What connection?  And how is it undermined?