What is the defence of inevitable accident in a motor vehicle accident case?

The defendant/applicant submits this is not one of those cases where the defendant is relying on the defence of inevitable accident arising from some factor under his exclusive control or wholly within himself as in the case of Perry v. Banno, [1993] B.C.J. No. 59, where the defendant claimed a sudden unanticipated loss of consciousness in which case the burden shifts to the defendant. The defendant contends this is a case where there is an external cause of the accident - the deer running into the road - and in light of that explanation which renders the accident as consistent with no-negligence as negligence, the plaintiffs have failed to prove their case.