elements are established, a defendant can escape liability by establishing the
defence.
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Some texts and case use the expression “affirmative defence”. This can mean
two things. In some jurisdictions such as the United States an affirmative
defence just means a defence supplied by a defensive element as distinct from
a defence based on a dispute over facts that constitute the positive elements.
In some jurisdictions, such as Australia, an affirmative defence refers to a
defensive element of a special kind. It is a defence where the responder has the
responsibility to prove the defence according to some standard of proof,
usually the preponderance of probabilities or the balance or probabilities as it
is also called. An example is insanity as a defence to a criminal charge.
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Non
affirmative defences on this classification are defences where the initiator, not
the responder, has the ultimate duty of rebutting the defence according to
some standard of proof. However, in these cases they are required to do this
only if there is some evidence of the defence before the court.
Listing Defensive Elements
There are two methods of setting out the defensive elements of a rule. One is
to list and label the positive elements then list and label the defences. The other
is to place all the elements in the same list and to indicate that an element is a
defence by specifying that it must be absent for the rule to apply. Each way is
equally valid so any choice between them is made according to personal taste
or functional convenience.
Nature of Elements
Since a rule is seeking to change the world, it has to identify and attach itself to
the part of the world that it will change. It has to mark out the events,
happenings, incidents, or transactions that it wants to affect. Lawyers call
these facts. A rule, therefore, has to designate a class or category of facts as
the target of its regulation.
A legal rule identifies these classes of facts by its elements. Each element
consists of a required fact for the rule to apply. It delineates a specific class or
type of fact. Elements in a legal rule are for purposes here labelled Element 1,
Element 2 and so on. Consequently, the range of elements for any rule
consists of Elements 1-n.
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For example Person A might have killed Person B in circumstances the
raise a prima facie case of murder. However, Person A might establish facts that
demonstrate that they killed Person B only because Person B was seeking to kill
them, and their only option in the circumstances was to kill Person B first. In
doing this they are raising a defence of self defence.
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R v Porter (1933) 55 CLR 182, 183-184
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