“We are unable, at certain times, to bring into our consciousness with sufficient force the memory of the suffering and humiliation of even a week or a month ago. We are without defense against the first drink.” (Chapter 2, Big Book)
“Once more: The alcoholic at certain times has no effective mental defense against the first drink. Except in a few rare cases, neither he nor any other human being can provide such a defense. His defense must come from a Higher Power.” (Chapter 3, Big Book)
‘At certain times’ suggests that sometimes I can rely on reason to keep me away from drink and sometimes I cannot. The danger is that the some-time effectiveness of the mental defence lulls me into believing I can rely on it continuously. I then start to rely on reason not God, and, when faulty reason tells me that a drink is a good idea, I will drink.