Animals are also very expert at this.
All through the child's second year, and longer, his sense of the persons around him is in this stage.

The incessant "why ?" with which he greets any action affecting him, or any information given him, is witness to the simple puzzle of the apparent capriciousness of persons.

Of course he can not understand "why"; so the simple fact to him is that mamma will or won't, he knows not beforehand which.

He is unable to anticipate the treatment in detail, and he has not of course learned any principles of interpretation of the conduct of father or mother lying back of the details.
But in all this period there is germinating in his consciousness--and this very uncertainty is an important element of it--the seed of a far-reaching thought.

His sense of persons--moving, pleasure-or-pain-giving, uncertain but self-directing persons--is now to become a sense of agency, of power, which is yet not the power of the regular-moving door on its hinges or the rhythmic swinging of the pendulum of the clock.