For example, you conclude "somebody has a hat" from "this guy has a hat". You do so via a rational transition. How is it made? What in the process by which the inference was made makes the inference rational? In this case it seems that what makes the process intelligent is that from the fact that a guy has a hat, it follows that somebody has a hat. But to understand intelligence, the question is how this relation is transparent to the system that makes the inference.
How do we know that the postman has brought the mail when the mailbox rattles, that the lamp is broken because a cable is loose, that a blender works by suckung the heavy stuff into a moving blade? How do we get to understad these things? What are the little mental moves that we have to make in order to be said to understand such facts?
Psychology uncovers many of these processes. A flashing light on the periphery of you visual field will pull your attention to the location of the flash. As a result, you are significantly faster in detecting things at that location as long as they appear a short period of time after the flash. However, if there is a delay between the flash and the target you become slower to detect it. This is called Inhibition of Return (IOR). IOR does not seem to be a quirck but one of those rational mental moves. If you have already looked at a place, it makes sense for you to inhibit that location least you keep looking on the same place.
Psychology trades on processes like this. It thus allows us to understand intelligence. It tells us about the small mental movements that make our thinking rational.
