People have the habit of dealing lightly with thoughts that come. And the atmosphere is full of thoughts of all kinds which do not in fact belong to anybody in particular, which move perpetually from one person to another, very freely, much too freely, because there are very few people who can keep their thoughts under control.
When you take up the Buddhist discipline to learn how to control your thoughts, you make very interesting discoveries. You try to observe your thoughts. Instead of letting them pass freely, sometimes even letting them enter your head and establish themselves in a quite inopportune way, you look at them, observe them and you realise with stupefaction that in the space of a few seconds there passes through the head a series of absolutely improbable thoughts that are altogether harmful.
Ref: Questions and Answers (1929 – 1931)