Sandi Metz

"This tension between the costs of concretion and the costs of abstraction is fundamental to object-oriented design. Concrete code is easy to understand but costly to extend. Abstract code may initially seem more obscure but, once understood, is far easier to change. Use of a duck type moves your code along the scale from more concrete to more abstract, making the code easier to extend but casting a veil over the underlying class of the duck."