> As a demonstration of the principle, consider two contradictory statements—"All lemons are yellow" and "Not all lemons are yellow"—and suppose that both are true.
I am not understanding why we are freely supposing both are true?
It's demonstrating the implications (principle of explosion) of a contradiction being allowed in a system of formal logic. You can change "suppose both are true" to "suppose the rules of a logical system permit stating both are true".
I am not understanding why we are freely supposing both are true?