How do you know “what you are thinking”?  You can’t know, there should only be one of you!  For you to know your own thoughts there would need to be someone there to think them and someone there to see them.  But you do!  You know what you are thinking.  In fact you can visualize ideas, concepts, even music, and you are quite aware of it.  How?  Who had the thoughts?  Who observed them?

It is possible to only be of one mind.  Philosophers have a name for this; it is called a philosophical zombie (http://en.wikipedia.org/wiki/Philosophical_zombie), though in true philosopher fashion this is widely though not to exist or to be impossible.  Most people assume that this is how animals live, only through stimulus-response behavior with no actual thought or awareness.  Most people seldom watch animals actually behave.

Perhaps this is because to really watch animals behave, and to study the mind of an animal one might realize that most of the animals around us have all of the brain parts that we have.  There are actually very few real differences between the brain of a cat, dog or cow or human.  Most animals have most or nearly all the major brain structures that humans have, the major difference is mainly the size of ours is bigger than most.

Most that is, though certainly not all.  An elephant has a brain that is 4.3  times larger (13 pounds vs our own 3 pounds) and a sperm whale’s is 7 times larger!  About the only thing truly special about  our much prized human brain is its size in comparison to our body weight, which is higher than most (though not all, hummingbirds have more).

So how do you know what you are thinking?  Well, one look at any reasonably normal human brain and you immediately can see its most obvious feature, the very large and wrinkled cerebrum, and that is distinctly divided into two obvious halves.  Each of these halves, the hemispheres, contain its own short term and long term memory processing ares, its own emotional areas, its own body sensing areas, its own motor areas and its own self awareness and judgement areas.  Each hemisphere has all of the parts needed for complete thought.

Children have actually had half of their brain removed, following injury, disease or illness and, when they are young enough the child has developed and learned to function almost completely normal.  Each hemisphere is a complete brain.  And connecting these two is a thick and complex band of nerves, the corpus callosum, that allows the two halves (in normal people) to communicate.

So, you know what you are thinking, and it is this ability that gives you self awareness and introspection, as well as allow you to multitask (within limits).