The Post Office Horizon scandal has highlighted two issues in the law of England and Wales. The first is that there is a common law Presumption (I use a capital “P” henceforth) that devices, including computer systems, are operating correctly unless there is evidence to rebut this assumption. The second is that evidence that would rebut such an assumption may fail to be disclosed in particular legal proceedings, and the Presumption may be invoked in those proceedings, even though evidence does exist to rebut it. This arguably happened in various prosecutions associated with the Post Office Horizon system and was explored in Phases 5 and 6 of the Williams Inquiry.

The issue whether the Presumption is appropriate has been explored inter alia by software reliability experts (including myself) in https://journals.sas.ac.uk/deeslr/article/view/5143 and we found it wanting. However, some have doubted whether the “main” issue in the Horizon scandal is misuse of the Presumption, or failure of disclosure, e.g., https://www.benthamsgaze.org/2019/12/19/resolving-disputes-through-computer-evidence-lessons-from-the-post-office-trial/#comment-136259 and, much more briefly, https://bsky.app/profile/k80bex.bsky.social/post/3lgazb36ec22o .

I think it well to point out that use of the Presumption can lead to inappropriate conclusions even when there is no failure of disclosure. This article argues that.