[papertyger]

Sure, but Drake’s equation is just a Fermi estimate - a reasonable guess about a matter we don’t have nearly enough data about.

"Reasonable" in what sense?

... but the fact is that we do know that intelligent life is allowed by the physics of this universe. Speculation about how often it occurs is certainly more legitimate than speculation about how common water flowing uphill on other worlds might be.

I don't see how until you've repeatably demonstrated how it in fact started here.

Life, any life, is a proverbial "turtle on a fence post."

We know the capabilities of turtles, and we know the characteristics of fence posts. All the calculations in the world are not goring to give you a legitimate probability for how often you can expect to find another turtle on another fence post.

[AnotherUnixGeek]

"Reasonable" in what sense?

In that these inferences don't violate or contradict any known facts and can be considered a valid possible consequence of those facts. Fermi estimates are often required in interviews in the software industry - it's a good way to find out how candidates approach and reduce problems. Without data, the results can't be confirmed of course, but the guess-timates often turn out to be quite good.

Life, any life, is a proverbial "turtle on a fence post."

So was the notion of stars having planets. Prior to verification over the last couple of decades, we had no way of knowing whether planets actually existed anywhere but here around our star. But what we could observe of our own star certainly warranted speculation. Detecting life will be far harder, but speculations about it's existence don't rank with speculations about physical impossibilities, IMHO.