"For the notion of causality to make sense we need to constrain the class of graphs that can be used in a causal model. Obviously, it’d make no sense to have lo

"For the notion of causality to make sense we need to constrain the class of graphs that can be used in a causal model. Obviously, it’d make no sense to have loops in the graph: We can’t have X causing Y causing Z causing X! At least, not without a time machine. Because of this we constrain the graph to be a directed acyclic graph, meaning a (directed) graph which has no loops in it." Of course you can have loops, no need for time machine: https://en.wikipedia.org/wiki/Virtuous_circle_and_vicious_circle https://en.wikipedia.org/wiki/Virtuous_circle_and_vicious_ci... https://en.wikipedia.org/wiki/Causal_loop_diagram https://en.wikipedia.org/wiki/Causal_loop_diagram https://en.wikipedia.org/wiki/Positive_feedback https://en.wikipedia.org/wiki/Positive_feedback