Couldn't the missing premise be A==>Not X?
The thread there could be
Not D==>Not Z==>A==>Not X
Not Z still comes before Not X
Not D still comes before Not Z
Not D still comes before A
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Because X>Z is also not z > not X so the logical flow would be not not d > not z > not x. Since A > not D and NOT not X then it would be not x > A
nimakian801January 10, 2019
If the missing premise was A > not x then the conclusion would be not D > not x. This is just how I understand it.
Nic_geraciSeptember 23, 2019
I'm not following this on either.. I had X - >Z - >Not A - >D = X - > Not A
IrinaSeptember 23, 2019
We are given the following premises:
(1) X-> Z (2) ~D -> ~Z (3) ? (4) Therefore, ~D-> A
Let's start by taking contrapositive of (2): (4) Z-> D
We can now combine premise (1) and (4) to form the following chain: X -> Z -> D resulting in the following inference:
(6) X -> D
Now how do we get from X ->D to the conclusion ~D -> A. Let's start by taking a contrapositive of (6) so that we have ~D in both conditional statements:
(7) ~D -> ~X
From here to make our conclusion follow logically, we need to have the following chain:
~D -> ~X -> A that is equivalent to ~D-> A.
Since we already have ~D-> ~X as one of the premises, the missing premise is ~X -> A.
