What is the test for a plea of non est factum under the principle of negligence?

He then dealt with the exception arising under the principle of non est factum and stated: “An anxious consideration of all the authorities referred to by counsel and in the courts below has brought me to the conclusion that, insofar, as Carlisle v. Bragg decides that the rule that negligence excludes a plea of non est factum is limited to the case of negotiable instruments and does not extend to a deed such as the one before us, we should refuse to follow it.”