Suppose:
If Zayd is human, he is an animal.
If Zayd is an animal, he is a body.
It does not follow that “If Zayd is a body, he is human.” Being human is sufficient for being a body, but being a body need not be sufficient for being human.
Following a sufficient condition forward differs from reversing it. A correct chain keeps the direction of implication intact.
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Conjunctive Syllogisms with Conditional Propositions
Suppose:
If Zayd is human, he is an animal.
If Zayd is an animal, he is a body.
It does not follow that “If Zayd is a body, he is human.” Being human is sufficient for being a body, but being a body need not be sufficient for being human.
Following a sufficient condition forward differs from reversing it. A correct chain keeps the direction of implication intact.