Final answer:
When observing the effect of an increase in X on Y with all other factors held constant, the assumption used is known as ceteris paribus, which allows isolating the relationship between the two variables.
Step-by-step explanation:
If we wish to observe the effect that an increase in variable X has on variable Y while assuming that nothing else is changing, then we are making the ceteris paribus assumption. This is a fundamental concept often used in economics and other social sciences to isolate the relationship between two variables by holding other potentially influential variables constant. Ceteris paribus, which is Latin for 'all other things being equal', is essential for causal inference because it allows us to hypothesize that any observed change in Y is due to changes in X alone. The assumption of "ceteris paribus" means holding other relevant factors constant or assuming that nothing else is changing except the variable (X) of interest. In this context, if we want to observe the effect of an increase in X on Y while keeping other factors constant, we are making the assumption of ceteris paribus.