As for “analogy based on resemblance,” if it is accepted, it falls within one of these two categories. The shared feature connecting the original case and the new case must be either the cause itself or something that entails the cause. Yet sharing something that entails the cause does not entail sharing the ruling; rather, the shared feature may or may not be accompanied by the cause. We therefore cannot know that the analogy is valid; indeed, it is valid only if both cases share the cause. We cannot know that they share it unless we know that they share either the cause itself or something that entails it, since the presence of what entails something necessitates the presence of what it entails. If we suppose that they share neither something that entails the cause nor the cause itself, the analogy is certainly invalid, since the cause would then be confined to the original case. If that is not known, the validity of the analogy is not known.

The validity of an analogy may also be known through the absence of any relevant difference between the original case and the new case, even when neither the specific cause nor its indication is known. The absence of any relevant difference necessarily means that they share the ruling. Since analogical reasoning is complete only through establishing the absence of a relevant difference or identifying a shared feature—a universal encompassing both cases that entails the ruling—and since either procedure can be cast in syllogistic form, this is not an inference that a ruling applies to one particular merely because it applies to another. Rather, it involves the ruling necessarily applying to the universal, and the universal necessarily applying to its particular instances. This is the very essence of syllogistic reasoning.

If it is asked, “How is it known that the shared feature entails the ruling?” the answer is: “By the same means through which the major premise of a syllogism is known.” Identifying the middle term amounts to identifying the shared connecting feature, while the major term necessarily applying to it amounts to the ruling necessarily applying to that shared connecting feature, as previously pointed out. One particular may also serve as evidence for another when they mutually entail one another, or when one entails