"Filtering", as a technical issue in writing, probably wins the prize for Most Useful Concept With Most Unhelpful Name (although, for that prize, "Free Indirect Style" is a very close runner-up). But John Gardner called it that in one of Creative Writing's founding texts, The Art of Fiction, and Janet Burroway sets Filtering all out very clearly in her classic Writing Fiction, so we're stuck with the label. The basic idea is that writers very often use phrases which get between the reader and a straightforward representation and evocation of what's happening. Gardner describes it as:
... the needless filtering of the image through some observing consciousness. The amateur writes: "Turning, she noticed two snakes fighting in among the rocks." Compare: "She turned. In among the rocks, two snakes were fighting ..." Generally speaking - though no laws are absolute in fiction - vividness urges that almost every occurrence of such phrases as "she noticed" and "she saw" be suppressed in favor of direct presentation of the thing see.
In place of Gardner's "amateur" (bless his trenchant/intolerant 1960s attitudes...), I would say "first-draft writer". That's not just because I'm kinder (more soft-hearted?): it reflects the realities of the writing process. As I was discussing in my post Filtering, Scaffolding and How To Perform an Explain-ectomy, all these extra, explainy, notes-to-self phrases come about as a very natural stage in drafting. Burroway explains:
As a fiction-writer you will often be working through "some observing consciousness." Yet, when you step back and ask the readers to observe the observer - to look at rather than through the character - you start to tell-not-show, and rip us briefly out of the scene.
Now, of course, there's nothing wrong with telling, in the right place, but Burroway's point is important. As she says, "cutting away filters is an easy means to more vivid writing", and it seems to me that this kind of "cutting away" is typical second-draft work. That's not to say that it always will be; as with so many technical things, as you work on it consciously, learn to spot it in others' writing and come to spot it more reliably in your own, you'll also find that you do it less often in first draft: conscious craft becomes intuitive craft.
Have a look at this, which is my riff on Burroway's example. She and Gardner assume an external narrator, narrating in third person:
Tony walked over to the sofa by the wall and sat crossly on it. He looked through the door and there, in the hall, he saw Jane's roller skates, lying yet again at the foot of the stairs. It seemed to him, though, that something was wrong with them. He noticed that one was standing straight but the other was at an odd angle, and then he saw that it was missing both its back wheels.
Take the "filtering" knife to the paragraph, and we get exactly the same event, and learn exactly the same things from it:
Tony walked over to the sofa by the wall and sat crossly on it. Through the door, in the hall, Jane's roller skates were lying yet again at the foot of the stairs. But something was wrong with them. One was standing straight but the other was at an odd angle, missing both its back wheels.
See what I mean? When I do a "filtering pass" on my own work - and I always, always do - it's like cleaning my glasses, or shooting in HD. When I cut away most "she saw", "he perceived", "they noticed", "I looked", everything about the storytelling is just the same, only better and brighter. That's not just because the padding - the words which insulate the reader from the immediacy and force of the moment - has gone: it's also, simply, that it's faster to read. We are drawn onwards more swiftly, through the setting up to the significant thing, the different thing, the thing which is going to propel the next bit of the story more quickly: what happened to Jane's skates?
If I do a search for each of my filter words in turn, I usually end up cutting about three-quarters of them and keeping the rest, and in re-jigging sentences, I make all sorts of other small improvements too - as Gardner does with the snake scenario. "She noticed two snakes fighting in among the rocks" doesn't have as much impact as "In among the rocks, two snakes were fighting", for more than one reason: a) The main verb phrase is no longer "she noticed snakes" but "snakes were fighting", and "fighting" has more energy and drama than "she noticed". b) In our end-weighted language, what a sentence ends with is specially important, and this one no longer ends with "rocks" but with "fighting".
And it's not just "saw", "noticed", "looked round" "watched" "observed" - the more straightforward, physical filter-words - that you need to be ruthless with. There's also what I think of as "thinking-filter-words", which I probably see even more often: the ones for thinking and feeling. Phrases like "I remembered that", "she wondered if", "they decided that", "we considered whether" "he thought about", "He thought back to when", "To her it seemed as if". They have much the same effect as the physical ones: a sort of padding, insulating the reader from the immediacy and the force of the moment. Riffing on Burroway's next example:
Cherry thought back to the days when she and Tony went skating together, though that was on ice. She remembered clearly how she'd hated the black hired boots, and recalled her happiness when he'd given her beautiful white ones for Christmas: now she felt sad to know that they'd never go skating together again.
Burroway's example has acquired those filter-words because it's a flashback: often the writer's nervously trying to keep the reader on track, knowing that this bit is about "then" not "now". But readers are experienced and sophisticated, and will intuit the move backwards with only a tiny bit of help:
She and Tony went skating together, though in those days it was ice. How she had hated the black hired boots! And how happy she'd been when he gave her beautiful white ones for Christmas. But now they'd never go skating together again.
Subject to what came before, "in those days", and the use of the past perfect - "she had hated" "how happy she'd been" - may well be enough help for us, without "thought back", and once we're in the past we stay happily there without hand-holds, until "now" brings us safely back. It's one of many excellent reasons for going on working with the past perfect, even though my sense is that it's fading out of colloquial use.
But it's not only in a flashback that thinking-filter-words creep in:
Cherry felt sad to think that they'd never go skating together again. She wondered if she should do anything with all those pairs of ice-skates, still stacked in the loft and, she guessed, covered in dust and probably rusty as well. She remembered that charity shop down on King Street, but she'd never noticed it specially, and had no idea if it wanted sports equipment to sell.
And notice particularly how crucial free indirect style is, in removing these filtering phrases. No need for "he thought" "she felt": just give us the thought, the feeling, in the narrative tense and person, and we'll get it:
They'd never go skating together again. Should she do anything with all those pairs of ice-skates? They were still stacked, covered in dust, in the loft, and probably rusty as well. Would that charity shop on King Street take them? But she'd no idea what they sold, or if they wanted sports equipment.
It's not a coincidence, either, that free indirect style is central to the whole business of psychic distance: those filter-phrases are information that the narrator is giving us: they're context and explanation, but here we don't need the information, and it's distracting to feel the the narrator's breath as they whisper the explanations in our ear.
That's not to say that "I looked and saw", "He felt that", "She seemed to be" are invariably a Bad Idea, of course (there's no such thing in writing, after all). There are times when that act of looking or feeling is the turning point: when you do, absolutely, want the reader to be aware of the observing/experiencing character's consciousness, not just the thing they're observing. But that's yet another reason for getting to grips with filtering, because then it's available when you need it specifically to control the reader's experience in this way:
Tony sat crossly down on the sofa by the wall. Through the door, in the hall, Jane's roller skates were lying yet again at the foot of the stairs. But something was wrong with them. One was standing straight but the top of the other was at an odd angle, missing both its back wheels. He got wearily up, looked very, very closely, saw spots of blood on the cuff, and felt a sick chill grip his stomach.
See how "looking" reinforces the Tony's act of looking, at this moment when everything for him changes. Mind you, you could argue with me about that last "felt", so let's try it without:
He got wearily up, looked very, very closely, and saw spots of blood on the cuff. A sick chill gripped his stomach.
I put the "felt" it in because I then could run the whole sentence straight on, as a series of steps forward, rather than stopping dead and starting again. It also echoes the syntax of the previous phrase - "saw spots of blood... and felt a sick chill". (My post about the advantages and pleasures of long sentences has more on this issue.) Different writers will feel it differently, but it's worth remembering that filtering words and phrases may earn their keep in your prose for reasons of sound and rhythm of the kind I explored here, and some of my examples in that post are relevant to this idea:
She put her hat on, tied her scarf tightly, remembered him in the pub, and realised he was an idiot.
As she climbed the hill and looked out over the stormy sea, suddenly, clearly, she realised he was an idiot.
There are filter-words in those - "remembered... realised" and "looked out ... realised", but they're earning their keep in both rhythm and meaning. It doesn't work in the same way at all to say
She climbed the hill. The sea was stormy. He was an idiot.
So there are important reasons for keeping filter-phrases in. Just make sure the effect is worth it - and cut the others.
ETA: And finally, thanks to Litlove's question in the comments, I've realised we should think about how filtering plays out with an internal, character-narrator, narrating in first person. The key to first-person narrators, I think, is to think about them as two separate entities:
a) the character who is an act-or in the story, and has the knowledge and limitations of any other character-in-action, and
b) the narrator-storyteller, who can tell the story however they like (including, if you choose, scenes they weren't there for, and knowledge they didn't have at the time.)
So, let's flip the pronouns and see what we get with the first example:
I walked over to the sofa by the wall and sat crossly on it. I looked through the door and there, in the hall, I saw Jane's roller skates, lying yet again at the foot of the stairs. It seemed to me, though, that something was wrong with them. I noticed that one was standing straight but the other was at an odd angle, and then I saw that it was missing both its back wheels.
We're very conscious of the "I"s there, aren't we, not least because all the sentences have the same basic, rather dull structure, starting with a subject + verb + object main clause. I see a lot of this in very beginnery novels and writers who don't have much skill yet in sentence-wrangling. (For some books to help with that, click here.)
Indeed, while the common assumption is that "first person" gets the writer and reader closer to a character (and it is a useful exercise for practising the closer-in psychic distances, even if you then flip the pronouns back to third person) the opposite can easily happen, and I see a lot of that too. With an external, third-person narrator, who can naturally be "outside" any particular head, it may be easier to ignore that filtering consciousness, and go "through the lens" to show us the action un-filtered. But with an internal, character-narrator, the writer is so conscious of the character's presence in the scene that all those filter-words slide in.
So what to do? It's easy. Take the "filtering" knife to the paragraph, and almost all the filter-words go, as do the pronouns:
I walked over to the sofa by the wall and sat crossly on it. Through the door, in the hall, Jane's roller skates were lying yet again at the foot of the stairs. But something was wrong with them. One was standing straight but the other was at an odd angle, missing both its back wheels.
Of course, it's still up to you how much of "I saw" etc., you let in, and how much you exclude. Some internal narrators are very chatty, explaining and glossing what they're telling, just as external narrators used to be, but these days are rather less so. Mr Dickens has nothing on Jacqueline Wilson's girls, or your average chick-lit narrator-heroine, for talking directly to the reader... But, again, having that extra context, explaining, adding, as supplied by the narrator, it is a slight distancing: insulation from the immediate moment. So, as with third-person, I'd suggest saving up the filtering - the sense of that narratorly consciousness, the presence of a lens - for the crucial moments when you want to emphasise that it's this character, now, who is experiencing this moment. And cut the others.