Excellent post, Bufty. (I've come to expect no less!)
I've shared this in many places. Once more with feeling.
Filtering occurs when the author tells the reader which of the senses the character used to gain information, or that s/he used the brain to reach conclusions, find correlations, etc. In good writing, the author trusts the reader to figure out a character knows there was a sound because she heard it, rather than telling the reader she heard it.
If the author filters everything through the point of view character, it creates psychic distance between what the character experiences and the reader. The sense of immediacy, of the reader feeling as if he's right there experiencing the scene as it happens, is diminished.
Instead of the author sharing the means by which the character experienced whatever she did, the author should cut directly to the experience, making it the subject of the sentence rather than the character (or a representative pronoun) being the subject. (Note that in first person narrative, removing filtering also removes a substantial use of the word "I," with considerable improvement.)
Filtering is the difference between
Susan heard the door creak. She wondered who was there. She noticed the scents of lavender and dust in the still air. “Grandma?” she said. She felt her heart race.
and
The door creaked. Who was there? Lavender and dust scented the still air. “Grandma?” Susan's heart raced.
That’s 30 words versus 17, and filtered-through-Susan distance versus immediacy.
Filtering can usually be spotted by the words that do it, although not all uses of these words are filtering. Search for knew, thought, considered, regarded, wondered, noticed, was aware, sensed, felt, saw, hoped, realized, smelled, heard and it seemed, looked like, appeared, was obvious/apparent. Decide on a case-by-case basis whether it's there to filter the point-of-view character's experience, and if it is, rewrite it.
Maryn, who will discuss furnace filters at a later time