Classical contradiction in logic I am studying for my logic finals and I came across this question:

Prove $((E\implies F)\implies E)\implies E$

I don't understand how there is a classical contradiction in this proof.
Using natural deduction, could some one explain why there is a classical contradiction in this proof?

Thank you in advance!

This proof follows the OP's attempt at a proof. The main difference is that explosion (X) is used on line 5:

The classical contradictions are lines 4 and 8. According to Wikipedia,

In classical logic, a contradiction consists of a logical incompatibility between two or more propositions.

For line 4, the two lines that show logical incompatibility are 2 and 3. For line 8, the two lines that show logical incompatibility are 2 and 7.
