Can You Assume the Converse in Mathematical Proofs?

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Discussion Overview

The discussion revolves around the question of whether one can assume the converse of a mathematical statement when proving it. Participants explore the implications of such assumptions in the context of mathematical logic and proof techniques, including reductio ad absurdum and the nature of circular arguments.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether it is permissible to assume the converse of a statement in proofs, noting that assuming what one is trying to prove is not allowed.
  • Another participant suggests that reductio ad absurdum is a valid method, implying that it allows for certain assumptions to be made to derive contradictions.
  • A different viewpoint is presented by a participant who mentions that constructivists may disagree with the elegance of proofs by contradiction, arguing that they can sometimes add unnecessary complexity.
  • One participant clarifies the distinction between converse and negation, stating that the converse applies specifically to conditionals and discussing the implications of assuming the converse when P and Q are equivalent.

Areas of Agreement / Disagreement

Participants express differing views on the validity and elegance of assuming the converse in proofs. There is no consensus on whether such assumptions are generally acceptable or advisable in mathematical reasoning.

Contextual Notes

Participants highlight the potential for circular reasoning when assuming the proposition being proved, and the discussion includes nuances regarding the conditions under which converses can be assumed without leading to circular arguments.

Kamataat
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I didn't want to post this in the logic subforum, because it's really basic...

I know that when proving mathematical statements, one can't assume what he/she is trying to prove. But can one assume its converse?

- Kamataat
 
Last edited:
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You mean like reductio ad absurdum...?It's by far the most elegant method of mathematics...

Daniel.
 
The constructivists may disagree that it is the most elegant, and indeed proofs by contradiction are to be frowned upon if they are unnecessary - often doing a proof by contradiction merely shows what we first started with to be true directly, and we've jist added pointless length to it.

You may assume not(X) is true and show that if so then we derive a logical contradiction, hence not(X) is false, X is true. Is that what you're getting at?
 
Do you mean converse or negation? I've only seen converse applied to conditionals: (Q -> P) is the converse of (P -> Q). Note that the reason you "can't" assume the proposition you're trying to prove is only that such an argument is circular; Circular arguments are still valid- they just don't tell you anything new. With one exception- when P and Q are equivalent- you can assume the converse of a proposition without being circular, since [(Q -> P) -> (P -> Q)] is a contingent proposition, i.e., [(Q -> P) therefore (P -> Q)] is an invalid argument; Its counterexample is [Q is false, P is true]).
 
Last edited:
matt grime said:
...

You may assume not(X) is true and show that if so then we derive a logical contradiction, hence not(X) is false, X is true. Is that what you're getting at?

Yeah, that's what I meant. Thanks!

- Kamataat
 

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