I like this analogy in the book (I paraphrase)
When we are hungry in the supermarket and pass the strawberries in the fruit and veg aisle, we may get the thought "I'm hungry, pick that up and eat it". But straight away, I dismiss the voice that tells me to do that. I don't steal. It is immoral.
In the same way, it is immoral for me to drink, because when I drink, I behave immorally. I drink drive (to the extent I don't know what route I took home) and I take money from my children.