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February 8, 1991

LUCY MERCADO, Individually and as Next Friend of BRIAN MERCADO, a Minor, Plaintiff,

James B. Zagel, United States District Judge.

The opinion of the court was delivered by: ZAGEL


 Is the testimony of an economist on the cash value of the lost pleasure of life admissible?

 We ought to begin inquiry into any proposed "scientific, technical, or other specialized knowledge," Fed. R. Evid. 702, by asking two questions. First, if the masters of the knowledge were to examine a set of facts, would all the masters come to the same conclusion? If they would their opinions are reliable. If the opinions can be verified or proven in some way the opinions are valid. If all structural engineers opine that the East Bridge will not stand against any hurricane, their opinions are reliable. If the bridge stands through the hurricane, their opinions are reliable but invalid. If the bridge falls with the first hurricane, their opinions are reliable and valid.

 Were complete reliability and validity the only hallmark of science, there would be precious little science. Not only is it difficult to achieve reliability and validity, it is often difficult to decide what shall constitute reliability and validity.

 Reliability is the simpler concept: It requires only that all experts would say the same things under the same circumstances. But who are the experts? Are all holders of Ph.D. or M.A. degrees in a given field experts? Or are all with "X" years of experience in the field, or all who pass a certifying examination experts? Who decides who are the experts?

 How do we test the proposition that all experts would hold the same opinion? These questions are answered by consensus.

 Life in a courtroom teaches there is very little reliability; equally qualified experts testify to propositions in utter contradiction. This teaching is flawed. Much science is reliable. Where nearly all who master some special knowledge are in agreement, their opinions are rarely heard in court. Agreement ends conflict, and the courtroom is an arena of conflict. Reliable opinions quietly enter courtrooms in the form of stipulations of fact, the uncontradicted testimony of the fingerprint expert, or the questioned document examiner. Or, in some cases, courts may belittle themselves by excluding reliable expert evidence such as courts did for too many years in paternity cases.

 Validity is a notion bristling with thorns and snares, among them the imponderable question of how we know anything with certainty.

 The provisional solution has been an agreement (or agreements) that verification of science is to be found in the accuracy of its predictions. A scientific principle is valid if its use tells us that some defined thing will occur and we perceive that it does occur. *fn1" The bridge falls in the hurricane. Sometimes prediction is subtler: A theory explains a past occurrence and predicts that we will, if we look in a certain place, find proof of this. The scientist looks for and detects the background radiation that would have resulted if the universe began with a big bang, as one theory proposes. This reliance upon prediction as validation comes from the positivist tradition in philosophy. This is today the accepted method for deciding what is science, what is scientifically or technically known. But it may not be a comprehensive standard for all belief and knowledge. *fn2"

 Prediction and its confirmation are often hard to find. Most scientific theories arise from the human thought in response to data perceived by imperfect human senses. Even data measured by machines is (and may always be) inexact. Validation becomes more difficult when science concerns itself more with things that we do not sense, things such as quarks, gravitors, and strange attractors, whose existence emerges at the end of a long chain of inference. A scientific theory is often proposed because it explains existing data. Explanation is not enough. Many discredited, even fanciful, theories explain what we have seen, heard, done, or felt. If explanatory power were enough to make science, then witchcraft, astrology, and paranoid delusions are science. *fn3" It is possible to provide many alternative explanations for a set of past events. One of the most profound challenges to the admission of psychiatric testimony is that psychiatric theory often has only the power to explain, and psychiatrists have expressed the view that they cannot and ought not to be predictors of human conduct. *fn4" Indeed, there are several explanatory psychiatric theories.

 Predictions that a bridge on Florida's southeast coast will fall in a gale is likely to be quickly and easily tested. But many scientific theories (and usually the most important ones) predict a very large number of events. We cannot be really sure of theory validity until all the theory's predictions are tested. Classical physics explained the physical world and predicted innumerable events. Its predictions were consistently verified. At the turn of this century, classical physics explained and predicted everything except black body phenomena and the result of Michelson-Morley experiments about the speed of light. To explain and predict these two phenomena, classical physics was ultimately subordinated to quantum physics and the theories of relativity. The point is that validation never ceases. New data might always explode accepted theory. Indeed we have never decided what minimum of validation is required for provisional acceptance. A single true prediction can destroy a competing scientific theory. A single true prediction will seldom validate a theory. Neither law nor science accepts the maxim veritas in uno, veritas in omnibus.

 The tentative nature of all theory has never stopped us from relying on theory because it is pragmatic to do so. Some knowledge is better than none. Classical physics still works quite well for ordinary mechanics. Astronomy went from a theory of spheres (Eudoxus) to a theory of circles *fn5" around the earth (Ptolemy) to a theory of circles around the sun (Copernicus) to a theory of ellipses around the sun (Kepler). Yet a sailor teaching a child the phases of the moon and navigation by the north ...

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