Peter Smith's rhetoric question on p. 155 of his An Introduction to Formal Logic (2 edn 2021)
(As so often, however, ordinary language has its quirks: so can you think of examples where a proposition of the form if A then C seems not to be straightforwardly equivalent to the corresponding A only if C?)
is acknowledged on p. 86 by Ernest Lapore in Meaning and Argument An Introduction to Logic through Language (Revised 2 edn).
All that is being said is that ‘only if ’ statements are conditionals. To see the difference between distinguishing statements on the basis of logical and nonlogical respects, consider pairs (26)–(29).
If water is boiled, it evaporates.
Water is boiled only if it evaporates.
My pulse goes up only if I do heavy exercise.
If my pulse goes up, then I do heavy exercise.
The even-numbered statements are grammatical (or acceptable), but the odd ones seem peculiar. [emphasis mine] (27) affirms straightforwardly that water being evaporated is a necessary condition of water being boiled. Intuitively, (26) in English seems to be less strong. It affirms that water evaporates when boiled, but it seems neutral as to whether it has to. Nevertheless, differences between the odd- and even-numbered statements do not establish that the odd statements should not be symbolized as the same material conditional. Consider the truth conditions for each statement. (26)–(29) are all false under exactly the same conditions: their antecedent is true, and their consequents false. So we will adopt the convention of symbolizing the members of each pair identically in PL. Although, given our convention, ‘⊃’ adequately symbolizes both ‘if, then’ and ‘only if ’ in PL, we should be careful not to presume that these expressions are equivalent in every respect.
But Lapore doesn't expound in simple English why "the odd ones seem peculiar"? Why can 27 and 29 be syntactically infelicitous or unnatural, but not their logically equivalents?
I already know, and am NOT asking about, why P if Q ≡ Q if only P.
