Yet another problem for identity theory is the possibility that other species feel pain. Whether these species are actual – such as fish and spiders – or hypothetical – such as aliens from Sirius – the problem is the same. Given that these species have very different ways of realising such a sensation as pain (that is, different physical processes for registering it), how can we assume that such an experience is identical with only a certain brain state?

There are two options here: either we assume that such creatures do not have similar experiences to us, or we admit that such conscious experiences as pain are “multiply realisable”. We shall come across this term shortly when we come to look at Functionalism, and it simply means that in theory a mind may depend upon vastly different physical mechanisms.