Step by step
The question
Question: are ¬¬p and p equivalent?
1 Rows for p
Each letter can be true or false.
One letters give 2 = 2 rows: every way they can go.
We make a table: a column for each letter, then a column for each part, from the inside out.
We write T for true and F for false.
p: the first half T, the second half F.
This way no row is missed.
2 Column for ¬p
We read this sign as not.
¬p is the opposite of p: T becomes F, F becomes T.
So it is T only in row 2.
3 Column for ¬¬p
¬¬p is the opposite of ¬p: T becomes F, F becomes T.
So it is T only in row 1.
4 Compare the columns
Now compare the two last columns.
¬¬p in red, p in purple.
Row by row they are the same: T, F and T, F.
Two nots cancel.
Careful!
To be equivalent they must match in every row.
One row that differs is enough to say no.
So they are equivalent.
We write ¬¬p ≡ p.
5 Check
Let's check row 1 by hand.
In row 1, p is true.
The first: ¬¬T gives T.
And the second: T gives T.
The same, as the table says.
Right!
Let's look back.
First: rows for p.
Then: column for ¬p.
Then: column for ¬¬p.
Then: compare the columns.
Last: check.