Some of the difficulties which have been felt in the application of
that power seem to me to be due to the circumstance that, unlike
most other powers conferred by s. 51 of the Constitution, it involves
the notion of purpose or object. In most of the paragraphs of s. 51
the subject of the power is described either by reference to a class
_ of legal, commercial, economic or social transaction or activity (as
trade and commerce, banking, marriage), or by specifying some class
of public service (as postal installations, lighthouses), or undertaking
or operation (as railway construction with the consent of a State),
or by naming a recognized category of legislation (as taxation,
bankruptcy). In such cases it is usual, when the validity of legis-
lation is in question, to consider whether the legislation operates
_ upon or affects the subject matter, or in the last case answers the
description, and to disregard purpose or object. An example will
be found in Victorian Stevedoring and General Contracting Co. Pty.
Ltd. v. Dignan (1). But "a law with respect to the defence of
the Commonwealth " is an expression which seems rather to treat
defence or war as the purpose to which the legislation must be
addressed. This peculiarity in the power has caused no departure
from the practice that excludes from investigation the actual
extrinsic motives and intentions of legislative authorities. But,
however it may be expressed, whether by the words - '" scope ",
"object", "pith", "substance", " effect" or " operation", the
connection of the regulation with defence can scarcely be other
than purposive, if it is within the power. No doubt it is possible
that the "purpose" here may be another example of what Lord
Sumner described as "one of those so-called intentions which the
_ law imputes ; it is the legal construction put on something done
in fact" (Blott's Case (2) ). For apparently the purpose must be
collected from the instrument in question, the facts to which it
applies and the circumstances which called it forth. It is evident
that among these circumstances the character of the war, its notorious
incidents, and its far-reaching consequences must take first place.
In some cases they must form controlling considerations, because
from them will appear the cause and the justification for the chal-
lenged measure. They are considerations arising from matters
_ about which, in case of doubt, courts can inform themselves by
looking at materials that are the subject of judicial notice.