The idea is that there are certain investments that give good returns. Governments can currently borrow at rates pretty close to 0%–even negative rates have pop

The idea is that there are certain investments that give good returns. Governments can currently borrow at rates pretty close to 0%–even negative rates have popped up here and there. And you would actually have to subtract inflation from that as well. So if some investment, say a new airport or funding for the National Science Foundation increases future growth even just slightly, it's perfectly fine to finance it with debt.