<p>Conditionals are ubiquitous in mathematics: we routinely express theorems using <i>universal conditionals</i> of the form ‘for all <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(x\)</EquationSource> <EquationSource Format="MATHML"><math> <mi>x</mi> </math></EquationSource> </InlineEquation>, if <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(A(x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> then <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(B(x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>B</mi> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>’. The logic of universal conditionals is underpinned by that of <i>propositional conditionals</i>, which take the form ‘if <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(A({x}_{0})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo stretchy="false">(</mo> <msub> <mi>x</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> then <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(B({x}_{0})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>B</mi> <mo stretchy="false">(</mo> <msub> <mi>x</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>’, where <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\({x}_{0}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>x</mi> <mn>0</mn> </msub> </math></EquationSource> </InlineEquation> is a specific object. In mathematics, propositional conditionals are subject to a <i>material conditional</i> interpretation: they are true unless <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(A({x}_{0})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>A</mi> <mo stretchy="false">(</mo> <msub> <mi>x</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is true and <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(B({x}_{0})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>B</mi> <mo stretchy="false">(</mo> <msub> <mi>x</mi> <mn>0</mn> </msub> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> is false. This, unfortunately, makes them peculiar in relation to natural language. Moreover, distinctions between propositional conditionals, universal conditionals, and implications are not always clear. How do introduction-to-proof textbooks deal with these issues? We address this question via a theoretically driven qualitative analysis of 17 texts commonly recommended at UK and US universities. We report on how these texts explain conditionals/implications, how they deal with the peculiarities of the material conditional, and how they discuss related language and reasoning. We then present a theoretical analysis of ambiguities that might leave a student confused, arguing that these arise due to the pragmatics of mathematical communication.</p>

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How do Introduction-to-Proof Textbooks Explain Conditionals and Implications?

  • Lara Alcock,
  • Rentuya Sa

摘要

Conditionals are ubiquitous in mathematics: we routinely express theorems using universal conditionals of the form ‘for all \(x\) x , if \(A(x)\) A ( x ) then \(B(x)\) B ( x ) ’. The logic of universal conditionals is underpinned by that of propositional conditionals, which take the form ‘if \(A({x}_{0})\) A ( x 0 ) then \(B({x}_{0})\) B ( x 0 ) ’, where \({x}_{0}\) x 0 is a specific object. In mathematics, propositional conditionals are subject to a material conditional interpretation: they are true unless \(A({x}_{0})\) A ( x 0 ) is true and \(B({x}_{0})\) B ( x 0 ) is false. This, unfortunately, makes them peculiar in relation to natural language. Moreover, distinctions between propositional conditionals, universal conditionals, and implications are not always clear. How do introduction-to-proof textbooks deal with these issues? We address this question via a theoretically driven qualitative analysis of 17 texts commonly recommended at UK and US universities. We report on how these texts explain conditionals/implications, how they deal with the peculiarities of the material conditional, and how they discuss related language and reasoning. We then present a theoretical analysis of ambiguities that might leave a student confused, arguing that these arise due to the pragmatics of mathematical communication.