係咪即係你上面條link入面講
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3. Problems for the Standard Theory
Given any two true sentences A and B, the conditional “If A, then B” is true. For example, provided it is true that the sun is made of gas and also true that elephants have four legs,
then the truth-functional conditional “If elephants have four legs, then the sun is made of gas” is also true. However, the gaseous nature of the sun would not normally be regarded as either a conceptually, or even a contingently, necessary condition of the quadripedality of elephants. Indeed, according to the standard theory, any truth will be a necessary condition for the truth of every statement whatsoever, and any falsehood will be a sufficient condition for the truth of any statement we care to consider.
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呢度所講噉樣?
我可唔可以噉理解呢,即係話classical propositional logic唔係全部都合理,但如果係因果關係apply落去,其實大部分都無問題嘅,但遇著上面呢啲咁奇特嘅cases,如果用必要條件去形容就會出現問題,例如係普通常識上嘅問題喇,因為用正常常識嚟諗,「太陽有輻射唔係1+1=2嘅必要條件」,但如果我哋apply充分必要條件落去classical propositional logic上面,佢就會話畀你知「太陽有輻射是1+1=2嘅必要條件」呢種咁荒謬嘅講法喇?
所以充分必要條件用喺classical propositional logic唔係完全成立,但有一部分係無問題