A major assumption in cardinal utility theory is that

The major assumption in cardinal utility theory is that utility is measurable. This means that it is possible to assign numerical values to the satisfaction or happiness that an individual derives from consuming a good or service. This allows economists to construct a numerical measure of utility and use it to analyze consumer behavior, including how much of a good they will consume at a given price, and how much they are willing to pay for it. Without this assumption, cardinal utility theory cannot be applied to real-world situations.