'No Emus on the Airplane!'
by John BatesStrict, first-order logic (think Mr. Spock) is extremely inadequate when faced with the real world. We are taught that logic is infallible, but it bears little resemblance to actual human reasoning.
A closer approximation to the way we actually think is a class of logics known as "nonmonotonic," which basically view all statements like "X is true" as really meaning, "X is true, unless there is some reason that it is not."
A quick example:
"All birds fly."
"Penguins are birds."
"Sparky is a penguin, therefore Sparky can fly."
"Penguins cannot fly."
"Sparky cannot fly."
"Airplanes can fly."
"Sparky is in an airplane."
"Sparky can fly."
One could revise this chain of reasoning to be represented as a first-order statement:
"If a bird is not a penguin or a bird is in an airplane, it can fly."
But what if Sparky was an emu?
We just don't reason that way: we don't develop immutable laws. We develop guidelines, and allow our beliefs to be revised in the face of contradictory evidence.
But there's a tension here: as Ken mentions in his comment to the post Unruly Photography, we are excellent at recognizing patterns. We are also very good at generalizing patterns into rules. And we have a distressing tendency to prefer to use existing rules rather than revise our beliefs. It is a survival trait: once we have accumulated a large amount of evidence that a generalization (read: "rule") is true, it can take a significant amount of contradictory evidence to shake that belief.
In general, you wind up with two classes of beliefs: let's call them "objective" vs. "subjective". An objective belief is one which is revised almost exclusively by means of exception: birds fly, unless they are penguins, or emus, or ostriches, or injured, or too young.
A subjective belief is more likely to be revised based on the context, such as Mike's statement that "Pictures which follow the rule of thirds too closely have an extra hurdle to cross to please me." Mike clearly has internalized a generalized anti-rule of thirds, but recognizes the nonmonotonic nature of his belief. (He doesn't like pictures that follow the rule of thirds, unless there is a reason that he likes them.)
The difficulty for photographers seems to be the difference between objective vs. subjective beliefs/rules. Once you have acquired the base rules, it is a natural tendency to assign them objective status. It takes a certain amount of sophistication and experience to push them back into the subjective class.
The fact that most people never make that leap is not necessarily a good reason not to recognize the general value of compositional "rules."
By the way, even though nonmonotonic logic as I described it sounds like a tautology, it is an extremely powerful tool.
Posted by: JOHN BATES
Mike comments: One of my favorite rules was expressed by photographer Frank DiPerna: "I absolutely never crop, except if I need to."
Cartier-Bresson's Behind the Gare St. Lazare, one of his pictures that is always cropped. The original picture was shot through metal bars, and there is a large black obsruction along the right side of the negative. It was this picture that Cartier-Bresson was sitting in front of when he told Charlie Rose that he never cropped his pictures. He was right; he almost never did.
P.S. How great is the internet? That's a stuffed emu on an airplane, above right. Picture by Catherine Warren. Stuffed emus demonstrably cannot, and can, fly.
P.P.S. Notice that the picture on the cover of the Second Edition of my book (below) beautifully observes the rule of two-fifths.