We are brought up communicating in language, but language fools us into thinking that we do know what THINGS ARE, that we comprehend what the abstract THING IS. Thus we conceive of our minds as being the cognitive THING that can be understood and implemented within the computer metaphor:
input a THING -> cognition as manipulating the THING -> output another THING.
Our own introspection leads us to the computer metaphor, but we feel that our introspection is horrible at comprehending what is really going on inside us. Nevertheless we persist with the computer metaphor, because it is a really difficult concept to get rid of.