Part II: Why some aspects of our consciousness cannot be described by science.
In part I of this post I argued that all our mental activity (our thoughts, feelings, sensory perceptions etc) must correspond to definite physical processes in our brain. But does knowledge of these physical processes tell us everything about consciousness?
Let me try to answer that by giving a well known example. A neuroscientist specializing in colour vision knows all about how the brain perceives colour. She knows for example everything about the physical changes that take place in a person's brain when he sees the colour red. The scientist has, however, been confined to a black and white room all her life. When she comes out of her room for the first time and sees the colour red one has to agree she learns something new, something that was missing from all her previous scientific knowledge.
In other words what it is like to see the colour red or to hear a particular sound is something beyond any scientific description. In fact these subjective experiences cannot be described in any kind of language. Let me elaborate by giving another well known example. Think of a person to whom everything red appears green and everything green appears red. Thus to him strawberries appear green (see picture on the right) and leaves appear red.
If you think about it he will never realize that his sensations are inverted with respect to the rest of us. This is because he will never be able to communicate to the rest of us the subjective sensation itself that he has when he sees something red or green. All he would be able to convey is that roses and strawberries have the same color or that leaves and grass have the same color. No one will of course ever dispute that.
Similarly what it is like to smell a flower or taste something salty can only be felt but not really described in words. These subjective experiences are called qualia (singular 'quale') by philosophers. Many would consider what it feels like to be angry or sad also to be a quale. To a scientist who wants to explain everything in terms of theories of matter qualia are a big, perhaps insurmountable, obstacle. The laws of physics do not tell us which physical events should have qualia or that qualia should exist at all.
These qualia- colours, scents, tastes, sounds etc are of course the fundamental building blocks of what we perceive as the external world. The world of our senses, however, is not the external world. The colours and sounds exist only in our heads and not in the world outside. In the world outside there is nothing qualitatively different between electromagnetic waves of wavelengths 350 and 450 nanometers but in the world of our senses the latter corresponds to blue light and the former cannot be seen. In the world of our senses the sensation of sound and the sensation of hotness/coldness are utterly different but in the world outside both are at a fundamental scale atoms and molecules in motion. The world that we perceive through our senses is nothing more than a simulation of the external world which encodes very limited information (that is important for our survival) about the world outside.
Aliens who have evolved under different circumstances may have very different sense organs and very different ways of perceiving the external world. The world that these aliens perceive would not be composed of images, scents and sounds but of the qualia related to their own senses to which we would be 'blind' and 'deaf'. We imagine everything in terms of images, scents and sounds so the world of their senses would be completely beyond our imagination. What would it feel like to perceive the world of their senses? Unfortunately no amount of scientific investigation of their brains will ever tell us the answer.
PS: (1) In the last example I did not need to talk about aliens. Many creatures, most notably bats, have sense organs which are very different.
(2) As is going to be the case with many other posts my knowledge on this subject is very elementary. It seems that this issue is far from settled among philosophers. There are counterarguments to many of the things that I have written here. I did not have the patience to go through these arguments but you can read them here and here.