Note: this obviously open to interpretation for this is just my thought on these terms, take it as you will... In Destiny, there is both paracausality and acausality. Before we continue, let's define the root word in both terms: causality. Causality is the notion that every cause has an effect. It's Newton's Third Law, every action has an equal and opposite reaction. Acausality is to be exempt from that rule. Paracausality is to be beyond/above that rule. For in-game examples, the Vex possess acausal power-- this is their ability to travel through time. Because they can time travel, they are exempt from the rule that every cause has an effect. However, because the Vex disregard the rules of causality, those rules need to exist in the first place. So ultimately, the Vex, for all their power, have inescapable, predictable limitations. They may rebel against the laws of physics, but if those laws do not exist, what is it exactly that they rebel against? Now paracausal power basically just rejects the system of causality altogether. Let's go back to the Books of Sorrow where the Vex fought an impossible war against the Hive for an example. The Vex had zero chance of winning because their power was acausal, whereas the Hive's power was paracausal. Think of their war as a game of basketball. The Vex, in order to win, would use their acausal power to go back in time and score an infinite amount of points across every moment in time. The Hive would use their paracausal power to basically, like, eat the idea of the game of basketball. If acausality is rebellion against the law that every cause must have an effect, paracausality is the destruction of that law. [i]-The One and Many[/i]