Bad thoughts?…. There can be several reasons for that. In fact there are several reasons. It may be due to a bad nature—if people have nasty feelings, these nasty feelings can be the cause of nasty thoughts.
It may be the opposite. Perhaps they are wide open to all sorts of suggestions from outside and, as I said, these suggestions enter them and gradually create nasty feelings.
It may be due to subconscious influences which are conflicting precisely because they are uncontrolled. When these influences rise to the surface, instead of being controlled and those which are undesirable refused, everything is allowed to enter as it likes, the doors are open. You are bathed in all kinds of things—good, bad, neutral, luminous, dark; it’s all there, and each one’s consciousness should, in principle, act as a filter.
You should receive only what you want to receive, you should think only what you want to think; and then, you should not allow these thoughts to be changed into feelings and actions without formal authorisation. In fact, this is the very purpose of physical existence. Each person is an instrument for controlling a certain set of vibrations which represent his particular field of work; each one must receive only the ones which are in conformity with the divine plan and refuse the rest.
Ref: Questions and Answers 1957 – 1958