People who are red/green color-blind cannot distinguish between green and brown. Gerald cannot distinguish between gr...

Irina July 31, 2019

@chris_va,

Could you specify which lecture are you referring to?

The argument in question has the following structure:

If P then Q
Q
_________
Therefore, P

you can also use notation with arrows:

P->Q
Q
_____
P

If a person is red/green colorblind (P) they cannot distinguish between brown and green (Q).
G cannot distinguish between green-brown (Q)
Therefore, G is red/green colorblind (Q)

The fallacy in the argument is known as affirming the consequent, making it invalid.

Let's compare it to the answer choices:

(A) If P (fair-skinned) then Q (sunburn)
P (fair-skinned)
Therefore, Q (sunburn)

This is a valid argument.

(B) If P (sinusitis) then Q (lose sense of smell)
Q(lose sense of smell)
Therefore, P (sinusitis)

Bingo. This argument commits the exact same fallacy as the one in the stimulus.

(C) If P (jaundice) then not Q (donor)
Q (donor)
Therefore, not P (not jaundice)

This is also a valid argument.

(D) If P (color-blind) then not Q (pilot)
P (color-blind)
Therefore, not Q (not a pilot)

This is again a valid argument.

(E) If P (diabetic) then not Q (large amounts of sugar)
P (diabetic)
Therefore, R (modified diet)

This is an invalid argument but it is structurally different from the stimulus.

Does this help? Let me know if you have any other questions.