Saturn and Mars are planets like the earth. They borrow light from the Sun and…

Concept — what makes an argument analogical

An argument is classified by the kind of link it claims between its premises and its conclusion. An analogical argument starts from resemblances that two things are already agreed to share and infers that they share one further property as well. The inference is inductive, not necessary: the conclusion is at best probable, and its strength depends entirely on how relevant the cited resemblances are to the property being carried across.

Application to this passage

1. The passage first grants resemblances between the Earth and the two other bodies: Saturn and Mars are planets, just as the Earth is.

2. It then adds two more shared properties: each shines by light borrowed from the Sun, and each revolves around the Sun.

3. From those resemblances alone it concludes that Saturn and Mars share a further property with the Earth as well, namely being inhabited by various orders of creatures.

Moving from "alike in these respects" to "therefore alike in this further respect" is precisely the pattern of an argument from analogy, and this passage is the classical textbook illustration of that form.

Cross-check against the other patterns of argument

• Deductive: the conclusion would have to follow with necessity, so that granting the premises made denial impossible. That each of these bodies orbits the Sun and shines by reflected light does not make the presence of living creatures unavoidable, so the link here is not one of necessity.

• Astrological: the reasoning would read the positions or motions of heavenly bodies as signs governing human character or earthly events. The passage draws no such influence.

• Mathematical: the conclusion would be reached by calculation or formal proof from quantitative premises. The passage states no quantities and performs no computation.

Result: the passage contains an analogical argument.

Worth noting: correctly naming the form does not make the argument a good one. The resemblances cited — orbiting the Sun and shining by reflected light — are shared by every planet in the solar system, including several on which no life has been found, so on their own they do nothing to establish habitation. The passage is unmistakably an analogy, and a weak one.