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Page Number: 33

2 

SMITH v. ARIZONA 

ALITO, J., concurring in judgment 

A 
Expert testimony presents a challenge for a legal system
like  ours  that  restricts  a  fact-finder’s  ability  to  consider 
hearsay.  This is so because an expert’s opinion very often 
is based on facts that are not proved in court.  As a modern 
treatise puts it, the value of experts lies in their ability to
“brin[g] to bear a body of knowledge largely extraneous to
the facts of the particular case.”  D. Kaye, D. Bernstein, A.
Ferguson, M. Wittlin, & J. Mnookin, The New Wigmore: Ex-
pert  Evidence  §1.2.1,  p.  4  (3d  ed.  2021)  (Kaye).    Wigmore
made the same point when he wrote that “[n]o one profes-
sional man can know from personal observation more than 
a minute fraction of the data which he must every day treat 
as working truths.”  1 J. Wigmore, Evidence §665(3), p. 762
(1904) (Wigmore).  Instead, experts routinely “rel[y] on the 
reported data of fellow-scientists, learned by perusing their 
reports in books and journals.”  Id., at 762–763 (emphasis
deleted); see also Kaye §4.1, at 165 (“[P]art of an expert’s
very expertise inevitably derive[s] from hearsay”). 

Despite  this  problem,  courts  in  Great  Britain  and  this 
country long ago recognized the value of expert testimony
and  concluded  that  they  “must  . . .  accept  this  kind  of
knowledge from scientific men,” even if it meant allowing 
testimony based on facts of which the expert did not have 
firsthand knowledge.  See 1 Wigmore 763; 1 S. Greenleaf, 
Evidence §430(l), p. 529 (rev. 16th ed. 1899) (“It would be
absurd  to  deny  judicial  standing  to  such  knowledge,  be-
cause all scientific data must be handed down from genera-
tion to generation by hearsay, and each student can hope to
test  only  a  trifling  fraction  of  scientific  truth  by  personal 
experience”); Slocovich v. Orient Mut. Ins. Co., 108 N. Y. 56, 
64, 14 N. E. 802, 805 (1888) (“An expert is qualified to give 
evidence as to things which he has never seen.  He may base 
an  opinion  upon  facts  proved  by  other  witnesses,  or  upon 
facts assumed and embraced within the case”). 

Recognizing this reality, a court in the late-18th century