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XIULU RUAN v. UNITED STATES 

ALITO, J., concurring
ALITO, J., concurring in judgment 

straightforward answer to this question.  The CSA contains 
an exception for prescriptions issued in the course of profes-
sional practice, and this exception is a carry-over from the 
CSA’s predecessor, the Harrison Narcotics Act of 1914, 38
Stat. 785.  In interpreting the Harrison Act, this Court held
that a registered physician acts “in the course of his profes-
sional practice” when the physician writes prescriptions “in 
good  faith.”  Linder  v.  United  States,  268  U. S.  5,  17–18 
(1925).  I would hold that this rule applies under the CSA 
and  would  therefore  vacate  the  judgments  below  and  re-
mand for further proceedings. 

The  Court  declines  to  adopt  this  approach  and  instead 
takes a radical new course.  It holds that the mental state 
expressed  by  the  terms  “knowingly  or  intentionally”  in 
§841(a)  applies  to  the  provision’s  “[e]xcept  as  authorized” 
proviso.  It bases this conclusion not on anything in the lan-
guage of the CSA, but instead on the “presumption, tracea-
ble to the common law, that Congress intends to require a 
defendant  to  possess  a  culpable  mental  state.”    Rehaif  v. 
United States, 588 U. S. ___, ___ (2019) (slip op., at 3). 

The Court’s analysis rests on an obvious conceptual mis-
take.  A  culpable  mental  state—or,  to  use  the  traditional 
Latin  term,  “mens  rea”—is  the  mental  state  an  accused 
must have in relation to the elements of an offense.  But the 
authorizations in the CSA that excuse acts that are other-
wise  unlawful  under  §841(a)  are  not  elements  of  the  of-
fenses  created  by  that  provision.    They  are  affirmative 
defenses.  The presumption that elements must be accom-
panied  by  a  culpable  mental  state—which  I  will  call  “the 
mens rea canon”—provides no guidance on what a defend-
ant must prove to establish an affirmative defense.  And for 
that  reason,  that  canon  does  not  help  to  decide  whether 
there is a good-faith defense in §841(a) prosecutions of phy-
sicians. 

The Court does not claim that the “[e]xcept as authorized”