Interfaces between objects should be defined clearly, and there should be as few as possible. So that dependencies may be exchanged easily.

Alias Law of Demeter

A method of an object may only call methods of:

• The object itself.
• An argument of the method.
• Any object created within the method.
• Any direct properties/fields of the object.

https://de.wikipedia.org/wiki/Gesetz_von_Demeter