Smoking Helps Me Think -- from Doctor Ernest Dichters The Psychology of Everyday Living (1947)
The mind can concentrate best when all outside stimuli have been excluded. Smoking literally provides a sort of smoke screen that helps to shut out distractions. This explains why many people who were interviewed reported that they cannot think or write without a cigarette. They argued that moderate smoking may even stimulate mental alertness. It gives us a focal point for our attention. It also gives our hands something to do; otherwise they might make us self-conscious and interfere with mental activity. On the other hand, our respondents admit that smoking too much may reduce their efficiency.
Depends on what the meaning of "is" is.