Let be the proposition:

"To visit two times the same place is forbidden".

It is forbidden to visit the same place three times?

A says:

 "No, because it is forbidden to visit the place two times".

B says:

"It is, because to visit the place three times you have necessarily to visit two times."

I think both answers are true.

  • I agree that it is personal opinion, because my first thought was, "It all depends on the circumstances." Who is doing the forbidding? Who or what is subjected to the rule? What is passing, what is it passing? A pendulum? I'm confused, and this seems more of a riddle than a logic problem. – Bread Apr 6 '19 at 20:18
  • I changed the verb "to pass". It is a pure logical question. The forbidding means "you just can't do it". – Carlitos_30 Apr 6 '19 at 20:39
  • B is right (I'm assuming that you have in mind one visitor only). What makes you think that A's answer survives B's argument? – Iñaki Viggers Apr 6 '19 at 21:05
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    It's a language thing, whether two times means exactly two times or at least two times. An ambiguity of natural language, not a genuine philosophical issue. – user4894 Apr 6 '19 at 23:31
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    @user4894 You're correct that there's an ambiguity of natural language. However, I don't think it matters in this case. Clearly, on the "at least two" reading the answer is B. But even on the "exactly two" reading, the argument in B still holds since visiting the place three times would require there to be a time (in between the second and third visits) at which you've visited exactly two, which is forbidden. So the answer is B anyway. – Ben W Apr 7 '19 at 4:31

Assuming that the prohibition is directed at one individual visitor, B is right. The absence of indications of temporary violation(s) being allowed defeats A's presumption that the prohibition refers only to the count of visits as of certain time/date.

For a person to visit the place thrice, that person necessarily has to violate the prohibition. Simply put, there is no such thing as a person's double-visit by which he can transition from having visited the place once to having visited it thrice.

By contrast, it is entirely feasible for a person to have one dollar and be paid two dollars with a two-dollar bill, whence he goes from having one dollar to three dollars without it being possible to identify an instant where he only had two dollars. Unlike the impossibility of a person's double-visit, there are two-dollar bills. Therefore, the counter-example formulated by @user4894 is mistaken.

In the dialogue of that counter-example ("Do you have two dollars? No, you have three dollars"), that response "No, you have three dollars" is actually answering the different question of "How much [or how many dollars] do you have?". But when a person has three dollars and is asked "do you have one/two/three dollars?", the correct answer in each case is yes. Similarly, when a person has visited the same place thrice and is asked "did you visit the place once/twice/thrice?", the correct answer is yes. And the affirmative in the questions with "twice" and "thrice" reflects that the prohibition has been violated.

The actual ambiguity in the OP's question is only whether:

  • the place is prohibited to be entered in more than once (regardless of whether that is by the same visitor or different ones); or

  • each individual visitor is prohibited to visit more than once.

B's answer reflects a conjecture that the prohibition is by person (not by place), whereas A's answer does not help to solve that ambiguity. Regardless, whatever conjecture(s) A and B may have are external and therefore irrelevant to what the actual question: the answer follows and never determines the question.

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