Necessity, Chance, or Design
When identifying the cause of things in everyday life—an object, an event, a signal from space—there are only three possibilities: 1) Necessity, 2) Chance, or 3) Design.
EVERYTHING is caused by necessity, by chance, or by design.
To say that something is caused “by necessity” requires that “it can happen in one and only one way.” In other words, only one possible outcome exists, and it must always occur.
The following are all examples of "necessity:”
- "Heads" come up while flipping a two-headed coin.
- 2 + 2 = 4.
- Water freezes at 32 degrees Fahrenheit.
- Massive bodies exert a gravitational force according to a specific formula.
In our universe, these events cannot have alternative outcomes. It is impossible for any other outcome to occur.
The opposite of necessity is contingency. Contingent outcomes do not have to occur, because multiple outcomes are possible. For example, while using a normal, six-sided die, rolling a “3” is contingent, since five other rolls are possible.
Contingent events are always the result of either chance or design.
For something to be caused “by chance” requires the event or outcome to be:
- Contingent: There must be more than one possible outcome.
- Plausible: There must be a reasonable probability of the outcome occurring “at random” or “by accident.” If the probability is exceedingly small, i.e. beyond plausibility, then “chance” is not an acceptable explanation.
In order to attribute something to design, the event or outcome must be: 
- Contingent: The must be more than one possible outcome.
- Complex: A complex event is one that has an extremely low probability of occurring by chance, so low that we consider it implausible. (Example: Flipping "heads" 100 times in a row using a fair coin.) High complexity and low probability go together.
- A computer password of just 2 uppercase letters is not complex, and could easily be hacked. There are only 676 combinations (= 26 x 26).
- But a password of 20 random digits, symbols, and letters is very complex. There are over 800 trillion trillion (= 8 x 10^26) possible outcomes using 52 upper- and lower-case letters, 10 digits, and 8 symbols. The probability of randomly obtaining a single specific outcome is so extremely low that it is implausible.
- Specified: Something is “specified” if it conforms to a specific recognizable pattern, or has evidence of structure. Something specified is more than just a random pattern; it matches a distinctive pattern that we recognize.
Example: Winning the Lottery
Here's a simple example of necessity vs. chance vs. design. Let’s assume that you win the lottery. Depending on the circumstances, your "win" could be the result of necessity, or chance, or design:
- Necessity: You were the only participant in the lottery; thus, you had to win, by necessity.
- Chance: There were many participants, the lottery was conducted fairly, and you just happened to win at random by chance.
- Design: Your friends rigged the lottery so that you were chosen the winner by design.
In general, we conclude that something is designed, or caused "by design," when we confirm that it is contingent (i.e. it didn't have to happen), and is both specified and complex. Said another way, we infer design when something matches a pattern that we recognize, and has a ridiculously low probability of having occurred by chance, e.g. naturally, or at random.
Simple examples include:
- Crop circles: We recognize their geometric patterns, and we know that weather and wind aren't plausible causes of such complex and precise shapes. Someone brought about those patterns by design.
- Mt Rushmore: We recognize the faces of the four images on Mt Rushmore, and we know that it is implausible that wind, erosion, and natural weather patterns could have constructed these specific images. Instead, someone brought about those patterns by design.
- Computer programs: Programmers recognize the structure and information contained in a computer program, and we know that monkeys typing at random on computers could never produce a functioning program. Computer programs are created by design.
Let's say you are shown three sets of scrabble tiles, and asked to determine if each set is the product of chance (Were these tiles randomly drawn from a bag?) or design (Did someone purposely arrange these tiles?).
- The three letters "c", "a", "t" in that order:
- The simple word “cat” is specified, because “cat” is a specific pattern of letters that forms a word we recognize in English.
- But it is not complex because it is only three letters in length; these three letters could plausibly be obtained, in the proper order, by drawing tiles at random from a bag.
- Because "cat" is not complex, we could not, with certainty, attribute it to design. (It could be produced either by chance or design.)
- An apparently random string of 100 letters: Any specific 100-letter string of letters has an extremely low probability of occurring, i.e. one chance in 26^100, which equals one chance in 10^141 (that's a 1 with 141 zeroes after it).
- The string of letters is complex, and we could never expect to duplicate a specific 100-letter pattern by drawing tiles at random.
- However, a string of random letters is not specified because it is not a recognizable pattern; it doesn’t form words or convey information.
- Because the string of 100 letters is not specified, we would not attribute it to design.
- An exact replica of a page from a Shakespeare play: A page from a Shakespeare play could contain perhaps 500-1,000 letters or more.
- A page of Shakespeare is specified because it contains specific words, sentences, and paragraphs, in the proper order, i.e. letters and structure, that are required for it to be a play by Shakespeare that we recognize.
- The specific sequence of letters also is complex, meaning it is implausible to have occurred at random. It will never happen that someone draws several hundred scrabble tiles at random in the specific order required to replicate a page of Shakespeare.
- Because the play is both specified and complex -- meaning that it matches a specific play we recognize, and it is implausible to have been produced at random-- we are correct to infer that the order of the tiles was caused "by design" (rather than occurred by chance).
On a fair roulette wheel, having the ball land on any specific value occurs “by chance” because the outcome is both contingent and plausible.
- Obtaining, say, a “24” on a roulette wheel is contingent because other outcomes can certainly occur.
- And a “24” is plausible; it occurs with a probability of 1 in 38 (since American roulette wheels contain 38 numbers, namely 1 through 36, plus 0 and 00).
- Over the course of an evening at a roulette table we would expect to see several spins result in a “24”. Nothing unusual about that.
But what if "24" came up repeatedly, or several times in a row? How many "24"s would need to occur IN A ROW for you to become suspicious? At what point would you conclude that someone somehow rigged the roulette wheel?
What if we saw “24” come up 3 times in a row? Is it reasonable to assume that it occurred by chance? Since the probability is getting small (i.e. 1/38 x 1/38 x 1/38, or 1 chance in 55,000), we might start to be suspicious of the roulette wheel or its operator.
What if a “24” came up 10 times in a row? Then we definitely would no longer attribute the outcome to chance. The chance of randomly obtaining a “24” ten times in a row is so extremely small that it can be ruled out as implausible. The game is definitely rigged.
- There is only 1 chance in 6.2 quadrillion (i.e. 6.2 million billion) of ten "24"s in a row occurring.
- On a fair roulette wheel we’re never going to see that happen, even if we played roulette all day, every day, for the rest of our life. (A lifetime of 70 years only contains 2 billion seconds. The chance of ten "24"s is one in 6 million billion!)
- In fact, if we played continuously for 500 million lifetimes, we would expect to see ten “24”s in a row occur only once! 
To summarize, all things are caused by one of three possibilities, as follows:
- By Necessity: When the outcome must occur as it did, because it is the only possible outcome.
- By Chance: When the outcome: i) did not have to occur, and ii) has a large enough probability of occurring "at random" or "by chance."
- By Design: When the outcome: i) did not have to occur, ii) is complex, i.e. has an extremely low probability of occurring "by chance", and iii) is specified, i.e. it matches a specific, recognizable pattern or structure.
 Dembski, Intelligent Design, 128-134. Much of this section is a paraphrase of Dembski.
 (1 spin per minute)(60 minutes / hour)(10 hours / day)(365 day / year)(50 years / lifetime)(500m lifetimes) = 6 quadrillion spins.