Locke also discusses a theory of the self. He writes: "the little and almost insensible impressions on our tender infancies have very important and lasting consequences." That is, the "associations of ideas" made when young are more significant than those made when mature because they are the foundation of the self—they mark the "tabula rasa". In the "Essay", in which he first introduces the theory of the association of ideas, Locke warns against letting "a foolish maid" convince a child that "goblins and sprites" are associated with the darkness, for "darkness shall ever afterwards bring with it those frightful ideas, and they shall be so joined, that he can no more bear the one than the other."