In order to detect faces, we need to understand what a face is, specifically what a human face is. Think about a typical human face. A typical human face has two eyes, a nose, and a mouth. But having these features isn't enough to define a human face. Dogs also have two eyes, a nose, and a mouth. We are, after all, products of mammalian evolution.
I encourage the reader to think more carefully about what makes a human face. We instinctively know what a face is, but to really quantify exactly what constitutes a face takes work. Often, it may lead to philosophical ruminations about essentialism.
If you watch terrible procedural TV shows, you might see faces being drawn with dots and lines when the detectives on TV are doing facial recognition across a database. These dots and lines are primarily due to the work of Woodrow Bledsoe, Helen Chan, and Charles Bisson...