The principle of causality
This is an idea that tells us that every event or action has its cause; for everything that happens, there is something that caused it. It's that simple and, it would seem, why is this needed?
It helps us understand and predict what will happen in the future and is the foundation for scientific research, experiments, and theory building. For example, in medicine they study the causes of diseases by analyzing risk factors and disease development. In physics, experimental confirmations of the theory of relativity demonstrate the causal relationship between mass and spacetime curvature. Economists can investigate how changes in interest rates (cause) affect inflation levels (effect).
In our daily life and at the macroscopic level, the principle of causality is fundamental and inviolable. But in quantum mechanics and relativity theory, there are theories that violate it, although they remain experimentally unconfirmed.
As a violation of this principle, one can cite the famous grandfather paradox. A time traveler returns to the past and kills his grandfather before he meets his grandmother. This makes the existence of the traveler himself impossible, which violates the causal relationship.