The IDEO Triad
Desirability, feasibility, viability. A product that hits two of three is a project, not a business. Score honestly on the lowest dimension.
Three lenses, all required: desirability (do people want it?), feasibility (can it be built?), viability (does it make money?). A product that hits two of three is a project, not a business. Most failed startups score nine out of ten on one lens and three on another — and pretend the average is six.
The discipline is to score honestly on the lowest dimension and treat that as the binding constraint. A beautiful, technically remarkable product that nobody wants is not "almost there." It's failing one of the three tests, which is the same as failing all of them.