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For more information, see the Wikipedia article:

Probability is a concept that is generally easy to understand intuitively, but can be difficult to define rigorously. When it is defined and studied carefully, it can lead to counter-intuitive results. Even elementary ideas of probability can be widely misunderstood, making it a good way to dishonestly bolster a weak argument.

When properly used, probability and its more "applied" cousin, statistics, can be powerful tools for discerning empirical truth. Indeed, essentially every field of modern science relies heavily on statistical analysis of data, which in turn relies on probability.

In the area of apologetics, ideas of probability lie at the heart of some arguments for the existence of God, especially the tornado argument and, to varying degrees, various cosmological arguments.

Definitions of probability

There are two main interpretations of probability, each of which leads to a different definition of probability:

Relative frequency 
The long-run relative frequency of occurrence of a random event (the fraction of the time it happens in a long run of repeated "trials") can be defined as the probability of the event.
Degree of belief (a.k.a., personal probability)
The degree to which one believes a statement to be true can be defined as the probability of that statement.

Although the degree-of-belief "definition" might seem quite weak, it can be made rigorous by carefully considering, for example, how much one would be willing to bet in a game where one would gain a certain amount of money if the statement turns out to be true. It can be shown that any internally consistent method of choosing one's wager must obey the laws of probability.

Conditional probability

For more information, see the Wikipedia article:

One of the most widely misunderstood concepts in probability has to do with conditional probability, the probability of one thing happening (or being true) given that something else definitely happens (or is true).

  • Example: "The probability that a 'king' has been drawn from a well-shuffled deck of playing cards given that you know it is a face card is one-third."

Conditional probabilities are often the result of narrowing the population of interest. Consider the following statements (using made-up figures):

  1. "10% of the population are atheists."
  2. "5% of the population are convicted criminals."
  3. "25% of convicted criminals are atheists."

Given this information, does it look like atheism is associated with criminal conviction? Well, 25% is much higher than either 10% or 5%, so it looks bad for atheists. But notice that the third percentage is calculated out of a different total: atheists instead of the whole population. That means the percents are not directly comparable.

The questions we really need to ask to get an accurate impression of what's going on are the following:

  • "What percent of atheists are convicted criminals?"
  • "What percent of theists are convicted criminals?"

(For simplicity sake, we'll assume that if you're not an "atheist", you're a "theist".)

Here's a table that matches the above information. All percents are out of the total population.

  Non-criminal Criminal Total
Theist 86% 4% 90%
Atheist 9% 1% 10%
Total 95% 5% 100%

More explanation to come...

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