… we can form a picture a human being in our heads, and say to ourselves ‘and it doesn’t have any phenomenal experience’. But this is far from conceiving of an exact physical duplicate of a human which lacks phenomenal experience.
Here’s an analogy, think of a mechanical clock, indeed, an exact duplicate of a mechanical clock you’re acquainted with. Can you conceive of the duplicate’s hands running anti-clockwise, rather than clockwise, or not running at all? You certainly could form a mental picture of the clock and say ‘and the hands run backwards’. But under close inspection, it’s not clear one could maintain this picture under scrutiny without making some change to the clock — say by rearranging the gears, or changing the direction of the motion imparted by the motor. …
What about the zombie case? Well, it’s clear we can conceive of the notion of ‘an exact duplicate of a human being’, and, separately, of the notion of ‘lacking phenomenal experience’. Conjoining the two doesn’t lead to an obvious contradiction … But it is far from obvious, given our relative lack of knowledge of the relationship between the mind and the body, whether or not a contradiction lies waiting to be unearthed in the notion of zombie.
The full post is available here.