Consider the image to the right. First, picture it as a duck. Now, as a rabbit. This image, made famous by the 19th century psychologist Joseph Jastrow, was invented to make a point about perception: That what we percieve is not only due to sensuous experience, but it is also due, in part, to our own mental activity. This point about psychology is however not as interesting as the philosophical implications that follow from it. E.g., that naive realism is false, and indirect realism (construed essentially as Kant stated it) is true. Let's take a look at how this line of thinking works.
Naive realism is the view that we percieve objects directly, where direct perception is taken to mean that there is no mediating representation, cognitive interpretation, or what-have-you that cuts us off from the thing-in-itself. What we percieve, for the naive realist, is essentially what would be there even if no one were there to percieve it (e.g., the thing-in-itself). In contrast, the claim of the indirect realist is that we always and only percieve a representation of a thing ("representation" being construed in various ways). That is, the object of our perception only is what it is, at least in part, because of our cognitive contribution to it, whether by way of a conceptual scheme, the fact that we only percieve Lockean secondary properties, or whatever.
So how does our beloved duck-rabbit bear on this debate? It's quite simple really: Consider carefully the nature of the optical illusion you just experienced. The raw sense data involved in the perception are identical both in the case of seeing the rabbit in the image (call this an "A-type" experience), and seeing the duck in the image (call this a "B-type" experience). Yet, the experiences (differentiating experiences by their differing qualia) are emphatically not identical. What follows is that, at least in this case, part of what constitutes a given experience as either A-type or B-type is your mode of cognition at that particular moment, not merely the object affecting the senses. If we percieved things directly, then the object itself would be sufficient to determine the type of experience we were having, yet it seems obvious that this isn't the case. Thus, we cognitively contribute to the world in such a way that we can have identical sensory-inputs, yet totally different mental-state outputs. It follows that direct, or naive, realism is false. Put in terms of an argument, this thought is as follows:
(1)An experience is individuated by either (a) the raw sense data involved, or (b) the conjunction of the raw sense data and the cognition involved.
(2) The experience of the duck-rabbit as either a rabbit, or a duck, cannot be individuated merely by the sense data involved.
(3) Therefore, the experience of the duck-rabbit as either a rabbit, or a duck, is individuated by the conjunction of raw sense data and our cognition.
Now here's a further question: What exactly is the nature of the cognition involved in this particular instance? I think the answer is that it is intentionality that is our cognitive contribution. What I mean to say is that, whenever we have an experience, the experience is never a bare experience, but it is always of something. The "of something" is key because it shows that all experience has intentionality embedded in it. We always take ourselves to be having an experience of a particular sort of object, but the idea that it is a particular sort of object shows that we take our experiences to have content. Take the following example: In the 1st century A.D., the wrist was considered to be a part of the hand. Nowadays, we view it as distinct (for us, the hand ends at the wrist). Now take a person from the 1st century, and the 21st century, and show them a picture of a hand and a wrist. The former will have the experience of seeing a hand, the latter will have a different experience, e.g. that of seeing a hand and a wrist. Two experience-types, one raw sense data. And what differentiates the two types? What they take their experience of the picture to be of, that is, the content of the experience provided by (you guessed it) our own cognition.
(As a sort of side-note, another interesting thing about the duck-rabbit experiment is this: A group of psychologists showed the picture to people on Easter Sunday, the vast majority of which percieved the picture as being of a rabbit, not a duck. Conversely, when participants were shown the picture during other parts of the year, such as October, most of them saw it as a duck. This shows that the type of experience we end up having is not only a function of our own cognition, but the context in which we find ourself doing the percieving.)
There's an interesting analogue here, I think, with Kuhn's idea of the so-called "theory-ladenness of observation" in science. Here we have what you might call the concept-ladenness of perception. As Kant said, we can only percieve that which we're able to cognize, and not vice versa. Of course, there are a number of sticky issues that make this whole debate not so simple, but I think this example is a good starting point for grasping the general idea of cognitive representation.
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