Humans notice causal chains when they're pretty young. This means, they start to figure out that when thing A happens followed by thing B, it is a common pattern, and if thing A happens followed by thing C it is probably coincidental. As they observe more closely, they start to understand the other elements in reality that are playing a role, and thus understand certain causal chains not just as "there's a pattern of correlation", but in a more detailed way: of seeing how that causal chain works and leads up to the observed effect.
This faculty is reason.