In his essay “Empiricism and the Philosophy of Mind” (1955-56), Wilfrid Sellars launches an attack against sense-datum theorists. I don’t want to defend sense-data because I think that’s a flawed concept, but I do want to point out a misstep that Sellars makes in this paper.
This objection has probably been made before, but for the sake of my notes I’m spelling it out as I read it.
In the paper, Sellars runs through a set of three propositions that he believes that sense-datum theorists must hold, but that Sellars believes is inconsistent.
A. X senses red sense content s entails x non-inferentially knows that s is red.
B. The ability to sense sense contents is unacquired.
C. The ability to know facts of the form x is Φ is acquired.
Sellars then states that “A and B together entail not-C; B and C entail not-A; A and C entail not-B” (132).
However, I think he makes a crucial misstep in his formulation here. Namely, in C Sellars refers to a fact, that knowledge “of the form x is Φ is acquired”, but I do not believe that what is stated in A truly takes this form.
On my reading of A, when we say that x non-inferentially knows that s is red, we are not saying that x knows the fact “s is red“. It may entirely be the case that x is unable to form the sentence “s is red”. Suppose that x has been living in Mary’s Room and is experiencing red for the first time — x would not be able to know that what x sees is red. Hence, x does not have knowledge of the form x is Φ in the sense that x can categorize his or her sense perception. But x still has some knowledge from this experience, and for the sense-datum theorist this is a recollection of that given sense data.
The difference here is a logical distinction between a de re and a de dicto formation. These two Latin phrases basically mean “of the thing” and “of what is said”. Symbolically, we can represent “x knows that” as Kx. We can also symbolize “α is red” as R(α).
Now, in the case of A, we have the claim that X senses red sense content s entails KxR(s). This is a de re claim — what x knows is that, of s, that object is of a certain colour, which we call red. The claim is that x knows that s has some property. We can argue a bit over whether this must be knowledge by acquaintance in the sense that Sellars talks about in this paper, but the idea here is that this is not knowledge of a fact, but rather the knowledge you would acquire whether or not you knew of the category “red” to begin with.
However, in C, we have the claim that knowing facts of the form Kx[Φ(α)]. This form translates in our example to Kx[R(s)]. This is the de dicto claim — what x knows here isn’t that s is red, but the sentence “s is red”.
So, the sense-datum theorist can simply agree that knowledge of de dicto facts in this form are acquired, but deny that A includes such a fact, since what is known can be expressed as a de re claim. Hence, the triad of propositions that Sellars attributes to the sense-data theorist is not inconsistent.
Sellars, Wilfrid. “Empiricism and the Philosophy of Mind”. Science, Perception and Reality. London: Routledge & Kegan Paul, 1963. pp. 127-164.