· Error of Affirming the Consequent We cannot conclude that “when B is true, A is true” simply because “when A is true, B is true.” This error is also known as the “conversion error.” For example: “Every time I go on vacation, I feel very relaxed. Therefore, when I feel very relaxed, I must be on vacation.” Feeling relaxed during a vacation does not mean that relaxation only occurs during a vacation; one might feel relaxed at home, while eating dinner, or while chatting with friends… Therefore, the fallacy of the above statement is obvious.
· Denying the Antecedent We cannot conclude that “when B is not true, A is not true” simply because “as long as A is true, B is true.” For example: “Every time it rains, the ground gets wet. Last night it didn’t rain, so the ground cannot be wet.” Undoubtedly, many situations can make the ground wet; it doesn’t necessarily have to rain.