When we previously discussed inductive reasoning we based our reasoning on examples and on data from earlier events. If we instead use facts, rules and definitions then it's called deductive reasoning.
We will explain this by using an example.
If you get good grades then you will get into a good college.
The part after the "if": you get good grades - is called a hypotheses and the part after the "then" - you will get into a good college - is called a conclusion.
Hypotheses followed by a conclusion is called an If-then statement or a conditional statement.
This is noted as
$$p \to q$$
This is read - if p then q.
A conditional statement is false if hypothesis is true and the conclusion is false. The example above would be false if it said "if you get good grades then you will not get into a good college".
If we re-arrange a conditional statement or change parts of it then we have what is called a related conditional.
Example
Our conditional statement is: if a population consists of 50% men then 50% of the population must be women.
$$p \to q$$
If we exchange the position of the hypothesis and the conclusion we get a converse statement: if a population consists of 50% women then 50% of the population must be men.
$$q\rightarrow p$$
If both statements are true or if both statements are false then the converse is true. A conditional and its converse do not mean the same thing
If we negate both the hypothesis and the conclusion we get a inverse statement: if a population do not consist of 50% men then the population do not consist of 50% women.
$$\sim p\rightarrow \: \sim q$$
The inverse is not true juest because the conditional is true. The inverse always has the same truth value as the converse.
We could also negate a converse statement, this is called a contrapositive statement: if a population do not consist of 50% women then the population do not consist of 50% men.
$$\sim q\rightarrow \: \sim p$$
The contrapositive does always have the same truth value as the conditional. If the conditional is true then the contrapositive is true.
A pattern of reaoning is a true assumption if it always lead to a true conclusion. The most common patterns of reasoning are detachment and syllogism.
Example
If we turn of the water in the shower, then the water will stop pouring.
If we call the first part p and the second part q then we know that p results in q. This means that if p is true then q will also be true. This is called the law of detachment and is noted:
$$\left [ (p \to q)\wedge p \right ] \to q$$
The law of syllogism tells us that if p → q and q → r then p → r is also true.
This is noted:
$$\left [ (p \to q)\wedge (q \to r ) \right ] \to (p \to r)$$
Example
If the following statements are true:
If we turn of the water (p), then the water will stop pouring (q). If the water stops pouring (q) then we don't get wet any more (r).
Then the law of syllogism tells us that if we turn of the water (p) then we don't get wet (r) must be true.
Video lesson
Write a converse, inverse and contrapositive to the conditional
"If you eat a whole pint of ice cream, then you won't be hungry"
