Last time we discussed the "split-brain" phenomenon, in which people with severe forms of epilepsy have had the two halves of their brains surgically separated.
This is done by cutting the bundle of nerve fibers that connects the hemispheres. The result is somewhat shocking, particularly to our assumptions about what it means to be an individual.
For example, using a computer, a split-brain patient may be shown an image of a church in the left half of each eye, and a blank screen in the right half. When asked what they saw, they will answer "nothing." That's because the visual information from the left half goes to the right hemisphere of the brain, which has no speech center, and thus can't say what it saw. With the communication lines severed, it can't tell the left hemisphere, which claims it saw nothing.
The question remains, did this person see the church or not? Can half your brain disagree with the other half?
Oddly enough, the split-brain patient did see something, it's just that the speaking half of the brain isn't aware of it yet. This can be demonstrated by asking them to draw a picture of what they saw. The non-speaking hemisphere can still control a hand for drawing. Surprisingly, the split-brain patient watching her own hand draw a picture and trying to figure out what she is telling herself.
Such experiments pose serious problems for the "common sense" notion of what we mean when we say, that we are a single person.