Theory will only take you so far.

The quote is from the movie Oppenheimer; they're discusing experimental vs. theoretical physics, and how sometimes, theory misses the mark.

Entrepreneurship is the same way.

It isn't called "theoretical business" but may as well be—margins, markets, competition, required rates of return. Broadly, this stuff defines what is and isn't possible. It's important.

But like in physics, theory will only take you so far—you have to "talk to customers" (YC), or "get out of the building" (Steve Blank). In business, the true test is whether they buy.