So one of the most important concepts in the signal processing is how do we define causal signals. Yeah so how do we do that? So let's get inspired again by our analog circuit here. So if you send in a delta pulse into that then we know it's a low-pass filter or response is probably something like that. Yeah so this is here the moment of our delta pulse. Here this is t equals zero and this is t equals zero here. So we are basically giving this system a small push with this delta pulse here. And then the system reacts to this here. That's the reaction of the system. Obviously before the delta pulse here this is zero. So this bit here is zero. For obvious reason because the system does not know that the delta pulse is happening. Yeah so the system is not a psychic predicting basically what's going to happen. And this means this system is called yeah so the system only evolves in time in this direction here. So a system evolves in time as it happens. So it reacts to these input signals here. So how do we do a mathematical definition of that? The mathematical convention is very simple. So the con the mathematical convention to tackle this is that we say that if we have a if this is our signal here and this is our function h of t and this is here t then the signal is always zero for negative time and only starts at zero. So we have something like this. And here it's this bit here it is zero. So this means the system is excited at the moment t equals zero and then it relaxes to this here. So it doesn't know anything what's happening before and so it only gets to now at the moment zero and then the system does something towards it. So this defines this in mathematics. So that this h of t is zero for t smaller than zero. That's our definition of causality.