Presenting evidence in court

1) Is the evidence based on a testable theory or technique;
2) has the theory or technique been peer reviewed;
3) in the case of a particular technique, does it have a known error rate and standards controlling
the techniques operation;
4) is the underlying science generally accepted
As a general guide these rules make a lot of sense, stressing scientific validity and the
competence of the expert witness, but without requiring consensus in the scientific community.
The problematic part is, however, who is to be the judge if the evidence complies with these
rules. In the US where Daubert has now been applied for some years the final word concerning
admissibility seems to have shifted from the scientific community to the trial judge who in most
cases is likely to possess little or no competence to evaluate scientific validity and reliability. As
may be expected this has not been without problems as I will explain.
Finally, I will say a few words about individualization. The problem of individualization has a
long history. One formulation of what it means for entities to be identical goes by the name of
The Identity of Indiscernibles often referred to as Leibniz's law. Leibniz first formulated the
principle in his Discourse on Metaphysics (1686): "it is not true that two substances may be
exactly alike and differ only numerically". In logical terms the law is often re-formulated as: if,
for every property F, object x has F if and only if object y has F, then x is identical to y. This is
also by and large the way we think about identity in forensic speaker identification although we
never come even close to the "for every property" requirement in any analysis. Two questions
arise when we thing about this law: 1) Is it true even in principle and 2) If it were can we even
approach fulfilment of the requirements by any practically applicable methods in forensic
science. Some scientist have argued that the principle is simply false, at least for some areas of
Quantum Physics (Cortes, 1976). Others (e.g. Saks & Koehler, 2008) have attacked the principle
on more practical grounds. Based on these and other studies, I will argue that we should abandon
the concept of identity altogether in the forensic context.
References
Cortes, A. (1976). Leibniz's principle of the identity of indiscernibles: A false principle. Philosophy of
Science, 43(4): 491–505.
Donnelly, P. (2005). Appealing statistics. Significance, 2(1), 46–48.
National Research Council. (2009). Strengthening Forensic Science in the United States: A Path Forward.
Washington D.C.: The National Academies Press.
Law Commission. (2011). Expert Evidence in Criminal Proceedings in England and Wales. (LAW COM No
325)
Leibniz, G. W. (1686). Discourse on Metaphysics. (on-line translation by G. R. Montgomery.
http://www.anselm.edu/homepage/dbanach/Leibniz-Discourse.htm)
Saks, M. J. and J. J. Koehler (2008). The individualization fallacy in forensic science evidence. Vanderbilt
Law Review, 61(1): 199–219.