Where an employee's dismissal is based on refusal of security clearance under s37(2) of the Defence Act, 42 of 2002, the dismissal constitutes incapacity arising from supervening impossibility of performance. However, such a dismissal can only be substantively fair if effected after completion of the procedural protections afforded by s39 and s41 of the Defence Act, which require provision of reasons for refusal, an opportunity for the employee to present information and make representations, and a review by the Personnel Security Review Board. The substantive basis for dismissal under s37(2) cannot be finally determined in the absence of reasons for the security clearance refusal and before completion of the review process. Until the review process is completed, the incapacity remains temporary rather than permanent, and does not warrant dismissal. Reinstatement is not reasonably practicable under s193(2)(c) of the LRA where an employee lacks the requisite security clearance that is a legal prerequisite for employment.