In a prosecution for misconduct in public office (misfeasance) where mixed motives are alleged, the Crown must prove that the power would not have been exercised but for the illegitimate purpose — a 'but for' causation test. It is not necessary to prove the improper purpose was the sole purpose, but directions using 'substantially motivated' or 'motivated to a significant degree' are inadequate because they invite the jury to weigh competing motives rather than apply the causation test. Trial judges must set out precisely what needs to be proved rather than relying on the good sense of the jury.
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