Simon Saunders is currently Professor of Philosophy of Physics at the University of Oxford, and Fellow of Linacre College. He is noted for his work on quantum mechanics (particularly the many-worlds interpretation), on identity and indiscernibility in physics, and on structural realism.

What do you think the idea of thinking the unthinkable actually amounts to? When we discuss “thought”, what do we actually mean?

I think it’s about limits to thought vis-a-vis something like states of affairs. To set it up, one might ask, “are there extraordinary facts about the universe that somehow are going to be forever beyond our comprehension?”

So the important question is whether or not we can have knowledge of every state of affairs? 

Right. I think there are other aspects to the question which are also interesting – issues of experiential knowledge etc. It comes up in the philosophy of mind: for example, qualitative experience called qualia. Is redness a property that we can have access to through science? Or is it something that we really have to directly experience ourselves?

The great example here is Mary, the colour-blind scientist. She knows everything there is to know about colour. She spends her life incarcerated in a black and white room, she communicates with a black and white video monitor. There are no mirrors. And then one day she leaves the room, and for the first time in her life she sees red. She thought she was learning something new. It was something she was incapable of getting at merely through a scientific understanding of colour – things like refractive properties of surfaces, electromagnetic structures, wavelengths of light, relations among wavelengths, that sort of thing.