A contract does not become unenforceable merely because something illegal is done in the course of its performance; the alleged illegality must go to the substance of the transaction and not be merely incidental or peripheral. Where illegality is not pleaded, a court will only raise it of its own motion where the transaction is ex facie illegal, the plaintiff cannot prove the case without proving illegality, or an incurable illegality clearly comes to light during trial. A party seeking to raise an illegality defence at a damages assessment hearing, when it could have been raised at the liability trial, will be estopped by the Anshun principle from doing so.
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