This is the cookie-cutter “ad hominem” flaw, where the author attacks the source of an argument rather than the argument itself. Deirdre doesn’t make this mistake; she attacks the philosophers’ argument, not the philosophers themselves.

Even if walking on the beach brings someone happiness one day and not the next, this isn’t a flaw in Deirdre’s reasoning. She just argues that because people sometimes feel happy walking on the beach, it’s untrue that achieving happiness always requires years of sustained effort.

The philosophers argue that it’s difficult to achieve happiness, which they define as the satisfaction of living up to one’s potential. Deirdre then argues that many people feel happy walking down the beach, but this is an entirely different meaning of the key term “happiness.”

The philosophers argue that “the goal of every individual is to achieve happiness,” but Deirdre never assumes that happiness is the goal of life. She just argues that the philosophers are too pessimistic in their argument about achieving happiness.

Deirdre doesn't mention the testimony of any group. Instead, she draws a conclusion about the philosophers’ argument based on a factual example about the feelings of many people when they walk on the beach.