The idea that a criminal defendant should, in a just social system, be able to mentally participate in their trial as well as be physically present dates back to the roots of English common law. Un-fitness to Stand Trial therefore depends upon the presence of a mental disorder at the of the trial. It is perfectly possible for a mental disorder to not affect Fitness to Stand Trial but to affect criminal responsibility as the latter would be relevant only at the time that the criminal offence was committed. Similarly, an individuals eligibility for involuntary hospitalization depends on different criteria again.
The criteria for Fitness have varied somewhat in the Canadian context, though at the present time, the criteria are laid out in the Mental Disorder section of the Criminal Code of Canada and were explored by the Ontario appellant courts in R v Taylor, 1992.
To be found Unfit to Stand Trial, the individual must, on account of mental disorder, be unable to conduct their defence at any stage of the proceedings before a verdict is rendered or to instruct counsel to do so. In particular, the unfit individual will be unable to understand the nature or object of the proceedings, the possible consequences of the proceedings or communicate effectively with counsel.