Steve,
When you move on to "knowing" you are evoking an
expected relationship...but its still a relationship. The word "lake" evokes a potential relationship without full perceptual engagement, but in order to understand this word (experience the evocation) without ever having had a perceptual experience we need to be in relationship with someone who has, via shared words/evocations.
LATER EDIT: We differ from ducks (

) by our use of language. Since "words" become disembodied signifiers of full perceptual experiences then we have the
impression of those experiences existing independently of ourselves as "objects". This is the fallacy of naive realism.
Patiodog.
This move by Descartes seems typical of philosophers who couldn't rock the boat. Newton was another, His disbelief in "the Trinity" was kept secret from the Cambridge authorities bcause it compromised his position at Trinity College.