The phrase "underlying premise" refers to the basic idea or belief that serves as the foundation for a particular argument or statement. It is the fundamental concept on which everything else is based. Full definition
One underlying premise of sector rotation strategies is that the investment returns of stocks from companies within the same industry tend to move in similar patterns. (fidelity.com)
Thermodynamically, this isn't reasonable, your apparent underlying premise is wrong. (skepticalscience.com)