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All the different kinds of probability 
are governed by the same basic rules. 

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A priori probabilities are governed by 
those statistical probabilities follow 

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these rules, subject to probabilities 
should follow these rules. 

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But what are these basic rules? 
We're going to look at four basic rules. 

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The first one governs simply negation. 
So you toss a coin and you get tails. 

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What's the probability that you get 
tails? 

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We said one in two or.5. 
Now, 

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what's the probability that you do not 
get tails? 

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Well, that means that's the probability 
that you get heads. 

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So it's also 5.. 
It's one minus the 5. probability of 

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tails gives you the.5 probability of not 
getting tails, that is, of getting heads. 

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Well, that's really simple, because there 
are only two possibilities. 

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What about a die? 
You roll a die and you get a two. 

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What's the probability that you'd get a 
two? 

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Well it's one in six. 
What's the probability that you would not 

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get a two? 
Well, not getting a two is getting either 

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one, or three, or four, or five, or six, 
so there are five possibilities of not 

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getting a two, and that means that, that 
five times out of six you do not get a 

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two. So the probability of not getting a 
two is one minus the probability of 

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getting a two. 
The probability of getting a two is one 

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in six, so the probability of not getting 
a two is five in six. 

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So our rule is basically that the 
probability that an event will not occur 

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is one minus the probability that the 
event will occur. 

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And we can put that in symbols like this. 
You put PR for probability and then in 

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parentheses you put what it's the 
probability of. 

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We're going to use H for hypothesis or 
the event that we're testing the 

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probability for. 
And the little squiggle or tilde means 

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not, 
just like it did when we were talking 

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about propositional calculus a few weeks 
ago. 

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So, the probability of not the 
hypothesis, is equal to one minus the 

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probability of the hypothesis. 
That's basically the rule for 

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probabilities of negations. 
And we explained why it holds, because 

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certainty is one. 
The thing happening and the thing not 

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happening have to add up to one, because 
it's gotta either happen or not happen, 

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so one minus the probability that it 
happens is gotta be the probability that 

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it does not happen. 
So hope now the rule makes sense, and 

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that you can apply it in a couple of 
exercises.