主句Although ., the Court in effect supported the medical principle of "double effect," ,尽管.但是
a centuries-old moral principle 指的是principle of "double effect," ,后面是补语补充说明
a good one that is intended and a harmful one that is foreseen 解释说明two effects
a centuries-old moral principle“补语主语” holding （认为）“补语谓语”that后面跟一个宾语从句
having two effects 修饰“an action”
an action (having two effects) is permissible( if the actor intends only the good effect)
If 后面是条件状语.