Sri Aurobindo says that all that one thinks one is, one can, by the very fact of that thinking, become. This knowledge of the fact that all that one thinks one can be, is a very important key for the development of the being, and not only from the point of view of the possibilities of the being, but also from that of the control and choice of what one will be, of what one wants to be.
This makes us understand the necessity of not admitting into ourselves any thought which destroys aspiration or the creation of the truth of our being. It reveals the considerable importance of not allowing what one doesn’t want to be or doesn’t want to do to formulate itself into thought within the being. Because to think these things is already a beginning of their realisation. From every point of view it is bad to concentrate on what one doesn’t want, on what one has to reject, what one refuses to be, for the very fact that the thought is there gives to things one wants to reject a sort of right of existence within oneself. This explains the considerable importance of not letting destructive suggestions, thoughts of ill-will, hatred, destruction enter; for merely to think of them is already to give them a power of realisation. …
One must not admit bad thoughts into oneself under the pretext that they are merely thoughts. They are tools of execution. And one should not allow them to exist in oneself if one doesn’t want them to do their work of destruction.
Ref: Questions and Answers 1956