# What kind of fallacy: "the centre of the solar system is the hottest, therefore each planet's core is the hottest."

Does the following argument involve a fallacy? If you think it does not, tick (d). If you think it does, which of (a), (b) or (c) might reasonably be construed as the fallacy in this argument?

The Solar system consists of the Sun and the nine planets. The hottest place in the Solar System is the Sun, which is a star located at its centre. Therefore, for each of the nine planets it is true that its hottest part is its core.

(a) undistributed middle (b) division (c) composition (d) no fallacy

• Is this a homework question? If so (even if not, actually), can you include your own thoughts on the answer? And also if so, it doesn't cease to perplex me how much value is given to being able to name individual types of fallacies. Sep 2 '16 at 5:00
• The fallacy is due to the fact that a planet is not a star. Sep 2 '16 at 8:35
• See Fallacy of division : "A fallacy of division occurs when one reasons logically that something true for the whole must also be true of all or some of its parts." "The Solar system [the whole] consists of the Sun and the nine planets. The hottest place in the Solar System is the Sun [true of the whole], which is a star located at its centre. Therefore, for each of the nine planets [the parts] it is true that its hottest part is its core." Sep 2 '16 at 10:22
• It is logically a fallacy of division (see above comment); but interestingly it is true analogically. Sep 2 '16 at 15:54
• @MauroALLEGRANZA: To see that it leads to wrong conclusion: The solar system consists mostly of vacuum, with a tiny bit of matter in the centre, and very few even tinier bits of matter circling around that centre. Therefore the planets including Earth consist mostly of vacuum, with a tiny bit of matter in the centre, and very few even tinier bits of matter circling around that centre. Sep 2 '16 at 21:09

Argument:

(i) The hottest part of the solar system is the Sun.

(ii) The Sun is located at the centre of the solar system.

(iii) Therefore, the hottest part of the solar system is its centre.

Using this reasoning, we assume it is true that:

(iv) The hottest part of any physical system is its centre

(v) The core is at the centre of a planet

(vi) Therefore, the core is the hottest part of the planet.

Similarly,

(1) The hottest part of any physical system is its centre.

(2) The glacier is at the centre of the lake.

(3) Therefore the glacier is the hottest part of the lake (which seems implausible)

The fallacy here is in the assumption that was derived from the Sun example. It is a fallacy of division, when one reasons logically that something true for the whole (the solar system) must also be true of its parts (other physical systems in the solar system).