Modus morons is another name for the logical inference rule
known as 'affirming the consequent
'. It takes the form of
If A then B
B, therefore A
In other words, if it is raining, then the ground is wet. The ground is wet, therefore it is raining.
To take a more fun example (extracted from Monty Python and the Holy Grail © 1977, Reed Consumer Books):
FIRST VILLAGER: We have found a witch. May we burn her?
ALL: A witch! Burn her!
BEDEVERE: Why do you think she is a witch?
SECOND VILLAGER: She turned me into a newt.
BEDEVERE: A newt?
SECOND VILLAGER: (after looking at himself for some time): I got better.
ALL: Burn her anyway.
BEDEVERE: Quiet! Quiet! There are ways of telling whether she is a witch.
BEDEVERE: Tell me ... what do you do with witches?
ALL: Burn them.
BEDEVERE: And what do you burn, apart from witches?
FOURTH VILLAGER: ... Wood?
BEDEVERE: So why do witches burn?
SECOND VILLAGER: (pianissimo) Because they are made of wood?
ALL: I see. Yes, of course.
BEDEVERE: So how can we tell if she is made of wood?
FIRST VILLAGER: Make a bridge out of her.
BEDEVERE: Ah ... but can you not also make bridges out of stone?
ALL: Yes, of course ... um ... er ...
BEDEVERE: Does wood sink in water?
ALL: No, no, it floats. Throw her in the pond.
BEDEVERE: Wait. Wait ... tell me, what else floats on water?
ALL: Bread? No, no no. Apples ... gravy ... very small rocks ...
BEDEVERE: No, no no.
KING ARTHUR: A duck!
(They all turn and look at ARTHUR. BEDEVERE looks up very impressed.)
BEDEVERE: Exactly. So .. logically ...
FIRST VILLAGER: (beginning to pick up the thread): If she ... weighs the same as a duck ... she's made of wood.
BEDEVERE: And therefore?
ALL: A witch!