The acquisition of data from main memory or from hard disk storage is usually one of the first steps in a forensic investigation. We revisit the discussion on quality criteria for "forensically sound" acquisition of such storage and propose a new way to capture the intent to acquire an instantaneous snapshot from a single target system. The idea of our definition is to allow a certain flexibility into when individual portions of memory are acquired, but at the same time require being consistent with causality (i.e., cause/effect relations). Our concept is much stronger than the original notion of atomicity defined by Vomel and Freiling (2012) but still attainable using copy-on-write mechanisms. As a minor result, we also fix a conceptual problem within the original definition of integrity.