The additivity principle states that we can add the effect or output of two systems together and the resulting combined system will be nothing more that the simple addition of each system’s output in isolation. For example, if one had two horses that could each pull a hundred kilograms of weight on a cart in isolation, well if we then combine these two horses to tow a single larger cart they would be able to pull twice as much weight. Another way of stating the additivity principle is that for all linear systems, the net response caused by two or more stimuli is the sum of the responses which would have been caused by each stimulus individually. The additivity principle is one of the conditions for a linear system.