Hi, I'm going to be posting an argument I've got from an acquaintance of mine against Direct Realism, I would like to know where this argument fails, if it fails, and how successful it is, if it is successful.


If we perceive green-ness, do we percieve a property of that object, which is greeness?

Assumptions:

(1) Universals can be each other. Example: Green could be the property of a surface. (reducibility/composition)
(2) A universal instantiated somewhere is the same as instantiated elsewhere. (universality)
(3) When we perceive an object as an idea we also have direct access to its properties. (naïve realism)
(4) It is possible to conceive of properties. (philosophical inquiry)


Let dSg be "y sees g's properties." and yPg be "y perceives g's properties". 
Let xCy=xSy (when we concieve something we see its properties.) (3)
g=s by (1) + (physical fact).
Conclusion ySs "y sees the surface". (=Conclusion 1)

It is possible to imagine the surface (4). 
Let dCs. 
dSs (3)
It is possible to perceive/conceive the surface as red. (moorean fact)
Let s=r. 
Then dSg and dSr by (4).
Which implies r=g. (=Conclusion 2)
If g=s (1). 
Then at all times we see green we see red by (2) + (Conclusion 1) + (Conclusion 2). 
This is absurd. 
Therefore:

If we reject (1) we get a promiscuous universe and we cannot reduce green to its physical state. Therefore no properties can be reduced to any other.
If we reject (2) we deny universality of properties, which is bad. 
If we reject (3) we reject naïve realism. (the most easy escape). 
If we reject (4) we've basically ruined philosophy.


Explication of argument's relation to Hume's scepticism.

(3') When we perceive an idea we have access to its properties. (realism)

Let g=s by (1) + (physical fact). 
Then with (3') we could infer:
xCy~xSy
So, when we conceive something we have some access to its properties. 
(let =/=> be "does not conclude")
xCy~xSy =/=>xCy=xSy. 
Therefore:
((dCy~dSy)&(g=s)&(dCy))=/=>(dSy)

Which means that we cannot reduce properties to each other/to our ideas. We cannot indicate "What's in the thing-in-itself." This "realism" has been reduced to total scepticism about properties of surfaces. 

The argument in everyday language.

Naïve realism:
Let Dennis see a green surface. Because the greeness of the surface is reducible to the structure of the surface, Dennis sees the propery of the surface.
But Dennis can also imagine the surface's structure in another colour, say red. 
This means that the structure of the surface in that instance is both red and green. 
Therefore red = green. And whenever Dennis sees something, and because universality, every time he'll see red, he also sees green. 

Hume's argument:
If Dennis sees a green surface. He cannot from that deduce that the structure of the surface is identical to the property of being green. Therefore Dennis cannot claim that he sees properties of the surface, but only mere green.


I'm doing this for my own benefit, for the most part, since I'm not entirely familiar with the literature concerning Direct Realism.