A computer without any input or storage devices is a boring creature. It can process information, but without an input device, none is being delivered to it. Without storage, it processes a stream of input and perhaps reacts (assuming some output device), but this stateless existence can never develop anything resembling intelligence.
Assuming memory is centered physiologically in the brain, a brain in a jar (without any sensory input) is limited to re-experiencing and processing that which it had encountered before being disembodied and placed in the jar.
Animals must have some kind of memory, as a domestic cat or dog remembers where its food is kept and recognizes its owners. They can learn certain behaviours and overcome certain obstacles. While they may associate certain abstractions with certain concrete items (eg. Pavlov's dog, a dinner bell, and food), it is assumed that they can't associate symbols with symbols. For example, a picture of a bell emblazoned on a box would not draw a dog to the box for nourishment, even though he might know a bell makes the sound which he associates with food.
However, humans can make complex assumptions based on the interaction of abstract symbols. Perhaps this is because we have a better developed and more complex memory, a larger part of the brain to store and abstractly associate symbols.
The more memory something can store, the more interactions one can perform between abstract notions, the more intelligent one is.
Personally, I believe the power of metaphor to be directly proportional to ones intelligence. The ability to adapt one cosmetically unrelated situation to another is extremely advantageous.
The facilities for such fluidity of thought, what makes one person more adept at making connections and associations, is better left to the neurobiologists. Speaking out of my ass, I'd say it has something to do with the alignment of chemical and electrical pathways in the brain. Perhaps varying levels of superconductivity in the organic flesh.
Now, wherein lies `the will?' Is there such a thing? We've set ourselves apart because we have this willful soul, but is it so unique? Most of our actions can be traced to learned behaviours or reactions to impulses. Apparently irrational actions might just be our desire to prod the environment, to gain input, like feelers on a bug.
What's our sense of self besides the ability to perceive our own thoughts? When I wonder what "I" am, I just get caught in a loop. I am my thoughts. I am myself inquiring about this phenomena which is my thoughts. The magic, the mystical feeling of self and soul, could be no more than a curiosity towards abstractions, akin to a dog sniffing out a hole in the ground.