10 lines
		
	
	
	
		
			451 B
		
	
	
	
		
			Markdown
		
	
	
	
	
	
			
		
		
	
	
			10 lines
		
	
	
	
		
			451 B
		
	
	
	
		
			Markdown
		
	
	
	
	
	
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tags:
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  - Logic
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  - propositional-logic
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  - derivation-rules
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This is also known as *proof by contradiction*. You start with an assumption declared in a subproof. If you can derive a contradiction from this assumption (typically from the introduction of another proposition and its negation), then you are permitted to derive the negation of the auxiliary assumption in the main proof. 
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