S cience, as traditionally conceived, is like interrogating the Greek mythological sea demon, Proteus. Poseidon commissioned him with the minor duty of protecting the seals of the oceans. More pertinent to the subject of our article is a quality Proteus possessed. He was omniscient in matters to do with the seas and could also modify his appearance to anything he wished, any animal, any plant, and even fire. Although elusive in his various appearances, it was possible with judicious interrogation and particularly with the aid of his daughter to extract the truth from him.A distinction made in the philosophy of science is whether theorists seek to discover truths or to create targeted frameworks that provide adequate prediction for some domain of phenomena and help to solve problems regarding that limited domain. The characterization of science as being like Proteus casts the goal of scientific theories as being to describe the world as it really is, independently of human expectations, perceptions, and measurements. Our aim here is to examine the development of scientific realism and instrumentalism in psychology and the implications of each for how we think about, formulate, and evaluate psychological theory.Scientists are, by and large, a practical lot, and it is sensible that readers might ask why any of this matters. Given numerous demands, it is understandable why scientists might adopt measures or methods without thinking exhaustively about the underlying assumptions of these operationalizations or how they fit within a broader set. Such constraints are the stuff of which apocryphal stories are fashioned by philosophers of science. More than 60 years ago, Eddington (1939), in his book A Philosophy of the Physical Sciences, told the story of a hypothetical scientist who sought to study the fish in the seas. The scientist wove a two-inch mesh net and commissioned a ship on which to sail the seas. Once on the high seas, this individual sailed to various sites, lowered the nets, hauled in a catch, measured and cataloged each fish, returned the catch to the deep, folded the nets, and sailed to another site to repeat the procedure. After several years of investigation, the scientist returned to announce there were no fish smaller than two inches in the seas. Taking the time to discuss the measures and methods we use, the underlying assumptions, their strengths and limitations, and the theoretical implications of these operationalizations may pay dividends in the long term in science.If the time scientists characteristically take to consider broadly what their measures mean is the equivalent of a brief gaze, the time taken by many to think about even more abstract philosophical issues such as scientific realism and instrumentalism may be the equivalent of a blink. Scientists tend to inherit rather than select these perspectives. This was certainly true of ourselves. But an inherited