Explanatory Gap

Functional versus Intrinsic Properties

To appreciate the force of the problem, it's necessary to distinguish between a mental state's functional properties and its intrinsic properties. The functional properties of a mental state, such as a visual experience, are determined by the role that that state plays in the overall functioning of the subject. So, for example, visual experiences of color carry information concerning the surface properties of the objects viewed and also contribute to determining the behavior of the subject in the relevant circumstances. By distinguishing red from green surfaces, we often thereby distinguish ripe from unripe fruit, which helps us decide whether it's advisable to eat what's in front of us.

Notice that a functional role is a rather abstract characterization that tells us, as it were, what job is done, but not how it's done, or what precisely it is that's doing it. We identify how a role is carried out by appeal to a state's intrinsic properties. In the case of the visual system, the bio-chemical properties of the neural circuits that fire in response to light stimuli explain how discriminations between red and green surfaces, for example, are made. This is similar to the way that descriptions of the hardware circuitry of a computer explain how the functional roles defined by the programs it's running are carried out.

Now we can see the relevance of the distinction for the explanatory gap. Consider again the qualitative character of an experience of viewing a bright orange leaf in the fall. The question is, what explains this particular qualitative character? Why is it like this and not some other way, to see the leaf? The obvious place to look for an answer is to the scientific story about the mechanisms from the retina through the visual system, how each neuron responds to the input from its neighbor upstream and transmits the relevant impulse to its neighbor downstream. But here is the problem. It seems that the most we can get from an account of the neural mechanisms in the visual system is an explanation of how the functional role played by a visual experience is carried out. In other words, a description of the physical properties of the neural mechanisms explains how they are caused to operate as they do and how their operations cause the downstream effects on cognition and behavior. It is unclear how knowledge of the physical mechanisms in the visual system can explain anything other than how they are caused and what they cause in turn.

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