If there was one thing that was really a pest for me this week, it was the "if and only if" statement. In English, you would normally use it to link a declaration with a condition, such that the declaration can only be made if the condition is right. For example:
I will eat apples if, and only if, it is already cut and peeled.
Most people will probably understand from this sentence that there exists a certain someone who is so lazy that he will not eat a fruit unless it is cut and peeled. Now, consider a scenario where "I" is in a room with nothing else but a table with a plate of cut apple sitting on the top. Will "I" eat the apple? According to CSC165, "I" will eat the apple. The reason being that in logic, the statement "I will eat apples if, and only if, it is already cut and peeled." is equivalent to saying something like "I will eat cut and peeled apples no matter what!". This is the same thing as saying that "if there exists cut and peeled apples, I must have eaten it". This sentence also suggests that no other person could have eaten the cut and peeled apple except of "I".
This has bugged me for the past several days now because to my knowledge "if, and only if" is used synonymously as "only if" in English. If you replaced the sentence above with "only if", then I believe that the sentence is trying to get across the same meaning. The other problem is that nothing in the sentence suggested that it is mandatory for "I" to eat the cut and peeled apple, and no other person can eat the cut and peeled apple. If they wanted to express that idea, then they could have said something like "I is the only possible person who will eat cut and peeled apple and no other apple." instead of using something confusing as "if, and only if".
At this point, the only way that I will be convinced that "if, and only if" is different from "only if" is if someone can give me a English grammar textbook and point out that they are actually different statements. Likewise, I cannot be bent to believe that "if, and only if" have a two-way implication function unless someone shows me that it can be used in such a way in real-life, not just in logic. Until then, unfortunate as it may be, I will have to brainwash myself into believing it for the assignment and next week's test. I would appreciate it if someone can post under comment section a link that "if, and only if" is just one of those grammatical thing rather than something logical.