"The problem may be expressed in the form of a dilemma that can arise for any psychological research that uses animals for the human case. If the animal subjects are not sufficiently like us to produce a model, then the experiments may be pointless [...]. But if the animals are enough like us to provide a model, it may be impossible to justify treating them in ways we would not treat humans. The researchers are caught in a logical trap: in order to defend the usefulness of research the must emphasize the similarities between the animals and the humans, but in order to defend it ethically, they must emphasize the differences. The problem is that one cannot have it both ways."
(James Rachels, "Created from Animals", p.220)