not making sense

I'm reading a book at the moment - one I got in a charity book shop, and one of the characters is a synaesthete - someone whose senses are mingled - for example they taste colours, see music, feel sound, hear pictures or smell temperature. I've never heard of this before and it really got me thinking about how we assume that everyone senses the same - but who's to say that when I smell a bacon buttie, it's exactly the same when someone else smells it. My mind is boggling way too much for a saturday night. I need to lie down.